Expressions formulas grade 7

starting number ⎯action⎯→resulting number A series of calculations can be described by an arrow string.. Write a problem that you can solve using arrow language.. Then write anotherarrow

Expressions and Formulas

Algebra

Mathematics in Context is a comprehensive curriculum for the middle grades

It was developed in 1991 through 1997 in collaboration with the Wisconsin Center

for Education Research, School of Education, University of Wisconsin-Madison and the Freudenthal Institute at the University of Utrecht, The Netherlands, with the

support of the National Science Foundation Grant No 9054928.

The revision of the curriculum was carried out in 2003 through 2005, with the

support of the National Science Foundation Grant No ESI 0137414.

National Science Foundation

Opinions expressed are those of the authors and not necessarily those of the Foundation.

Gravemeijer, K.; Roodhardt, A.; Wijers, M.; Kindt, M., Cole, B R.; and Burrill, G (2006)

Expressions and formulas In Wisconsin Center for Education Research & Freudenthal

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Development 1991–1997

The initial version of Expressions and Formulas was developed by Koeno Gravemeijer,

Anton Roodhardt, and Monica Wijers It was adapted for use in American schools by

Beth R Cole and Gail Burrill.

Wisconsin Center for Education Freudenthal Institute Staff

Research Staff

Thomas A Romberg Joan Daniels Pedro Jan de Lange

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Gail Burrill Margaret R Meyer Els Feijs Martin van Reeuwijk

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Fae Dremock Mary S Spence Martin Kindt

Mary Ann Fix

Revision 2003–2005

The revised version of Expressions and Formulas was developed by Monica Wijers and Martin Kindt

It was adapted for use in American schools by Gail Burrill.

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Contents v

Section A Arrow Language

Section B Smart Calculations

Section D Reverse Operations

Section E Order of Operations

Return to the Supermarket 40

Dear Student,

Welcome to Expressions and Formulas.

Imagine you are shopping for a new bike How do you determine the size frame that fits your body best? Bicycle manufacturers have aformula that uses leg length to find the right size bike for each rider

In this unit, you will use this formula as well as many others You willdevise your own formulas by studying the data and processes in thestory Then you will apply your own formula to solve new problems

In this unit, you will also learn new forms of mathematical writing.You will use arrow strings, arithmetic trees, and parentheses Thesenew tools will help you interpret problems as well as apply formulas

to find problem solutions

As you study this unit, look for additional formulas in your daily lifeoutside the mathematics classroom, such as the formula for sales tax

or cab rates Formulas are all around us!

Sincerely,

T

Th hee M Ma atth heem ma attiiccss iin n C Co on ntteex xtt D Deevveello op pm meen ntt T Teea am m

1 How old is the bus driver?

2 Did you expect the first question to ask about the number of

passengers on the bus after the fifth stop?

3 How could you determine the number of passengers on the bus

after the fifth stop?

When four people get off the bus and seven get on, the number ofpeople on the bus changes There are three more people on the busthan there were before the bus stopped.

4 Here is a record of people getting on and off the bus at six bus

stops Copy the table into your notebook Then complete the table

5 Study the last row in the table What can you say about the

number of passengers getting on and off the bus when youknow that there are five fewer people on the bus?

For the story on page 1, you might have kept track of the number ofpassengers on the bus by writing:

of calculations is called arrow language You can use arrow language

to describe any sequence of additions and subtractions, whether it isabout passengers, money, or any other quantities that change

7 Why is arrow language a good way to keep track of a changing

total?

Ms Moss has $1,235 in her bank account She withdraws $357

Two days later, she withdraws $275 from the account

8 Use arrow language to represent the changes in Ms Moss’s

Arrow Language

A

Number of Passengers Getting off the Bus

Number of Passengers

5 9 16 15 9

8 13 16 8 3

3 more

5 fewer

Arrow Language

Kate has $37 She earns $10 delivering newspapers on Monday.She spends $2.00 for a cup of frozen yogurt On Tuesday, she visitsher grandmother and earns $5.00 washing her car On Wednesday,she earns $5.00 for baby-sitting On Friday, she buys a sandwichfor $2.75 and spends $3.00 for a magazine

9 a Use arrow language to show how much money Kate

has left

b Suppose Kate wants to buy a radio that costs $53 Does she

have enough money to buy the radio at any time during theweek? If so, which day?

Section A: Arrow Language 3

Wandering Island

Wandering Island constantly changes shape On one side of the island,the sand washes away On the other side, sand washes onto shore.The islanders wonder whether their island is actually getting larger orsmaller In 1998, the area of the island was 210 square kilometers (km2).Since then, the islanders have recorded the area that washes awayand the area that is added to the island

11 What was the area of the island at the end of 2001?

12 a Was the island larger or smaller at the end of 2003 than it was

10 How deep was the snow on Friday

afternoon? Explain your answer

Year Area Washed Away (in km 2 ) Area Added (in km 2 )

1999 2000 2001 2002 2003

5.5 6.0 4.0 6.5 7.0

6.0 3.5 5.0 7.5 6.0

Arrow Language

A

Arrow languagecan be helpful to represent calculations

Each calculation can be described with an arrow

starting number ⎯action⎯→resulting number

A series of calculations can be described by an arrow string

10 ⎯⎯→
6 16 ⎯⎯→
3 19 ⎯⎯→
3 22 ⎯⎯→ 4 18

Airline Reservations

There are 375 seats on a flight to Atlanta, Georgia, that departs onMarch 16 By March 11, 233 of the seats were reserved The airlinecontinues to take reservations and cancellations until the planedeparts If the number of reserved seats is higher than the number

of actual seats on the plane, the airline places the passenger names

on a waiting list

The table shows the changes over the five days before the flight

Date Seats Requested Seats Cancellations Total Seats Reserved

3/11 3/12 3/13 3/14 3/15 3/16

233 47

51 53 5 16

0 1 0 12 2

Section A: Arrow Language 5

1 Copy and complete the table.

2 Write an arrow string to represent the calculations you made to

complete the table

3 On which date does the airline need to form a waiting list?

4 To find the total number of reserved seats, Toni, a reservations

agent, suggests adding all of the new reservations and then

subtracting all of the cancellations at one time instead of using

arrow strings What are the advantages and disadvantages of

her suggestion?

5 a Find the result of this arrow string.

12.30 ⎯
1.40⎯⎯→ ⎯⎯ ⎯ 0.62⎯⎯→ ⎯⎯ ⎯
5.83⎯⎯→ ⎯⎯ ⎯ 1.40⎯⎯→ ⎯⎯

b Write a story that could be represented by the arrow string.

6 Write a problem that you can solve using arrow language Then

solve the problem

7 Why is arrow language useful?

Juan says that it is easier to write 15
3  18  6  12
2 14 than

to make an arrow string Tell what is wrong with the string that Juan

wrote and show the arrow string he has tried to represent

At most stores today, makingchange is easy The clerk justenters the amount of the purchaseand the amount received Then thecomputerized register shows theamount of change due Before computerized registers, however,making change was not quite sosimple People invented strategiesfor making change using mentalcalculation These strategies arestill useful to make sure you getthe right change.

B

Smart Calculations

Making Change

1 a When you make a purchase, how do you know if you are

given the correct change?

b Reflect How might you make change without using a calculator

or computerized register?

A customer’s purchase is $3.70 The customer gives the clerk a $20 bill

2 Explain how to calculate the correct change without using a pencil

and paper or a calculator

It is useful to have strategies that work in any situation, with or without

a calculator Rachel suggests that estimating is a good way to begin

“In the example,” she explains, “it is easy to tell that the change will bemore than $15.” She says that the first step is to give the customer $15.Rachel explains that once the $15 is given as change to the customer,you can work as though the customer has paid only the remaining $5

“Now the difference between $5.00 and $3.70 must be found The difference is $1.30—or one dollar, one quarter, and one nickel.”

Section B: Smart Calculations 7

These methods illustrate strategies to make change without a computer

or calculator They are strategies for mental calculations, and they can

be illustrated with arrow strings

Another customer’s bill totals $7.17, and the customer pays with a

Arrow language can be used to illustrate the small-coins-and-bills-firstmethod This arrow string shows the change for the $3.70 purchase.

6 a What might the clerk say to the customer when giving the

customer the amount in the third arrow?

b What is the total amount of change?

c Write a new arrow string with the same beginning and end but

with only one arrow Explain your reasoning

Now try some shopping problems For each problem, write an arrowstring using the small-coins-and-bills-first method Then write anotherarrow string with only one arrow to show the total change

7 a You give $10.00 for a $5.85 purchase of some cat food.

b You give $20.00 for a $7.89 purchase of a desk fan.

c You give $10.00 for a $6.86 purchase of a bottle of car polish.

d You give $5.00 for a $1.76 purchase of pencils.

A customer gives a clerk $2.00 for a $1.85 purchase The clerk is about to give the customer change, but she realizes she does nothave a nickel So the clerk asks the customer for a dime

8 Reflect What does the clerk give as change? Explain your strategy

In problem 7, you wrote two arrow strings for the same problem Onearrow string had several arrows and the other had only one arrow

9 Shorten the following arrow strings so each has only one arrow.

Section B: Smart Calculations 9

making them shorter or longer

10 For each of the arrow strings, make a longer string that is easier

to use to do the calculation Then use the new arrow string to findthe result

a 527 ⎯⎯→
99 ?

⎯⎯ b 274 ⎯⎯→ 98 ⎯⎯?Change each of the following calculations into an arrow string withone arrow Then make a longer arrow string that is simpler to solveusing mental calculation

11 a 1,003 – 999 b 423 + 104 c 1,793 – 1,010

12 a Guess the result of this arrow string and then copy and

complete it in your notebook

273 ⎯⎯→ 100 ?

⎯⎯ ⎯⎯→
99 ⎯⎯?

b If the 273 in part a is replaced by 500, what is the new result?

c What if 1,453 is substituted for 273?

d What if 76 is substituted for 273?

e What if 112 is substituted for 273?

f Use one arrow to show the result for any first number.

Numbers can be written using different combinations of sums anddifferences Some of the ways make it easier to perform mental calculations To calculate 129
521, you can write 521 as 500
21 and use an arrow string

129 ⎯⎯→ 150
21 ⎯⎯ ⎯⎯→ 650
500Sarah computed 129
521 as follows:

129
500

⎯⎯→ ⎯⎯ ⎯⎯→ ⎯⎯
20 ⎯⎯→ ⎯⎯
1

13 Is Sarah’s method correct?

14 How could you rewrite 267 – 28 to make it easier to calculate

using mental computation?

Smart Calculations

Sometimes an arrow string can be replaced by a shorter string that iseasier to calculate mentally

⎯⎯ ⎯⎯→ ⎯⎯
64 ⎯⎯→ ⎯⎯ becomes ⎯⎯
36 ⎯⎯→ ⎯⎯
100Sometimes an arrow string can be replaced by a longer string thatmakes the calculation easier to calculate mentally without changingthe result

⎯⎯ ⎯⎯→ ⎯⎯ becomes ⎯⎯  99 ⎯⎯→ ⎯⎯
1 ⎯⎯→ 100

or

⎯⎯⎯⎯→ ⎯⎯ becomes ⎯⎯ 99 ⎯⎯→⎯⎯  100 ⎯⎯→
1The small-coins-and-bills-first method is an easy way to make change

Complete each of the following arrow strings

Section B: Smart Calculations 11

2 For each of these arrow strings, either write a new string that will

make the computation easier to calculate and explain why it is

easier, or explain why the string is already as easy to calculate

3 Write two examples in which a shorter string is easier to

calculate mentally Include both the short and long strings for

each example

4 Write two examples in which a longer string would be easier to

calculate mentally Show both the short and long strings for each

example

5 Explain why knowing how to shorten an arrow string can be

useful in making change

Write an arrow string that shows how to make change for a $4.15

purchase if you handed the clerk a $20.00 bill Show how you would

alter this string if the clerk had no quarters or dimes to use in making

change

Tomatoes cost $1.50 a pound Carl buys

2 pounds (lb) of tomatoes

1 a Find the total price of Carl’s tomatoes.

b Write an arrow string that shows how

you found the price

At Veggies-R-Us, you can weigh fruitsand vegetables yourself and find out how much your purchase costs You select the button on the scale that correspondsto the fruit or vegetable you are weighing

The scale’s built-in calculator computes the purchase price and prints out a small price tag The price tag lists the fruit or vegetable, the price per pound, the weight, and the total price

The scale, like an arrow string, takes theweight as an inputand gives the price as

an output

⎯⎯→

⎯⎯→

weight price

2 Find the price for each of the following weights of

tomatoes, using this arrow string:

3 a Write an arrow string to show the calculation for green beans.

b Calculate the price for 3 lb of green beans.

The Corner Store does not have a calculating scale The price

of tomatoes at the Corner Store is $1.20 per pound Siu bought

tomatoes, and her bill was $6

4 What was the weight of Siu’s tomatoes? How did you find

your answer?

In some cabs, the fare for theride is shown on a meter Atthe Rainbow Cab Company,the fare increases during the ride depending on thedistance traveled You pay abase amount no matter howfar you go, as well as a pricefor each mile you ride TheRainbow Cab Companycharges these rates.

Formulas

C

The base price is $2.00

The price per mile is $1.50

5 What is the price for each of these rides?

a stadium to the railroad station: 4 miles

b suburb to the center of the city: 7 miles

c convention center to the airport: 20 miles

The meter has a built-in calculator to find the fare The meter calculation can be described by an arrow string

6 Which of these strings shows the correct price? Explain

your answer

number of miles ⎯⎯⎯→ $1.50 ⎯⎯ ⎯
$2.00⎯⎯→ total price

$2.00 ⎯⎯⎯
number⎯⎯⎯⎯of miles⎯⎯→ ⎯⎯ ⎯⎯⎯→ $1.50 total price

number of miles ⎯ $2.00⎯⎯→ ⎯⎯ ⎯⎯⎯→ $1.50 total price

Taxi Fares

The Rainbow Cab Company changed its fares The new prices can befound using an arrow string.

number of miles ⎯⎯⎯→ $1.30 ⎯⎯ ⎯
$3.00⎯⎯→ total price

7 Is a cab ride now more or less expensive than it was before?

8 Use the new rate to find the fare for each trip.

a from the stadium to the railroad station: 4 miles

b from a suburb to the center of the city: 7 miles

c from a convention center to the airport: 20 miles

9 Compare your answers before the rate change (from problem 5)

to those after the rate change

After the company changed its rates, George

slept through his alarm and had to take a cab

to work He was surprised at the cost: $18.60!

10 a Use the new rate to calculate the distance from

George’s home to work

b Write an arrow string to show your calculations.

The arrow string for the price of a taxi ride shows how to find theprice for any number of miles

number of miles ⎯⎯⎯→ $1.30 ⎯⎯ ⎯
$3.00⎯⎯→ total price

Section C: Formulas 15

C

Formulas

Measure and record the following:

• the total height of a cup

• the height of the rim

• the height of the hold

(Note: The hold is the distance from the bottom

of the cup to the bottom of the rim.)

Make a stack of four cups and measure it Wasyour guess correct?

Formulas

11 Calculate the height of a stack of 17 cups Describe your

calculation with an arrow string

12 a There is a space under a counter where cups will be stored.

The space is 50 centimeters (cm) high How many cups can

be stacked to fit under the counter? Show your work

b Use arrow language to explain how you found your answer.

Sometimes a formulacan help you solve a problem You can write aformula to find the height of a stack of cups if you know the number

of cups

13 Complete the following arrow string for a formula using the number

of cups as the input and the height of the stack as the output.number of cups ⎯⎯→ ⎯⎯? ? ⎯⎯→ ? height of stack

Suppose another class has cups of different sizes The students use aformula for finding the height of a stack of their cups

number of cups  1

⎯⎯→ ⎯⎯ ⎯⎯→ ⎯⎯ 3 ⎯⎯→
15 height of stack

14 a How tall is a stack of 10 of these cups?

b How tall is a stack of 5 of these cups?

c Sketch one of the cups Label your drawing with the correct

height

d Explain what each of the numbers in the formula represents.

Now consider this arrow string

number of cups  3

⎯⎯→ ⎯⎯ ⎯⎯→
12 height

15 Could this arrow string be used for the same cup from problem 14?

Explain

We can write the arrow string as a formula, like this

number of cups  3
12  height

16 Could the formula in problem 13 also be used to solve the

problem?

17 These cups will be stored in a space 50 cm high How many cups

can be placed in a stack? Explain how you found your answer

Section C: Formulas 17

You have discovered some formulas written as arrow strings On thenext pages, you will use formulas that other people have developed.

Bike shops use formulas to find the best saddle and frame height foreach customer One number used in these formulas is the inseam

of the cyclist This is the length of the cyclist’s leg, measured in centimeters along the inside seam of the pants

The saddle height is calculated with this formula

inseam (in cm)  1.08  saddle height (in cm)

18 a Do you think you can use any numbers at all for inseam

length? Why or why not?

b Write an arrow string for the formula.

c Use the arrow string to complete this table.

d How much does the saddle height change for every 10-cm

change in the inseam? How much for every 1-cm change?

Formulas

C

saddle height inseam

height

frame height

Bike Sizes

Inseam (in cm)

Saddle Height (in cm)

50

60 64.8

70

80

Formulas C

To get a quick overview of the relationship between inseam lengthand saddle height, you can make a graph of the data in the table Inthis graph, the point labeled A shows an inseam length of 60 cm withthe corresponding saddle height of 64.8 cm For plottingthis point,64.8 is rounded to 65

19 a Go to Student Activity Sheet 1 Label the point for the inseam

of 80 cm with a B What is the corresponding saddle height inwhole centimeters?

b Choose three more lengths for the inseam Calculate the

saddle heights, round to whole centimeters, and plot the

points in the graph on Student Activity Sheet 1.

c Why is it reasonable to round the values for saddle height to

whole centimeters before you plot the points?

If you complete the calculations accurately, the points in the graphcan be connected by a straight line

20 a Go to Student Activity Sheet 1 Connect all points in the graph

with a line

b If you extend your line, would it intersect the point (0, 0) in the

bottom left corner? Why or why not?

c A line goes through an infinitenumber of points Does everypoint you can locate on the line you drew provide a reasonablesolution to the bike height problem? Explain your reasoning

21 Write a question you can solve using this graph Record the answer

to your own question Exchange questions with a classmate.Then answer the question and discuss with your classmate.Look at the formula for the frame height of a bicycle

inseam (in cm)  0.66
2  frame height (in cm)

22 a Write an arrow string for the formula.

b Complete the table (round the frame height to a whole number)

and draw the graph for this formula on Student Activity Sheet 2.

Margit used the formula to find the first two frame heights in the table.She did not round the heights Then she used the first two values to

20 40 60 80 100 120 140

50

60

70

80

c How might Margit find the frame height for an inseam of 65 cm?

24 a If you connect all points in this graph and extend your line, does

the line you drew intersect the origin (0, 0)? Why or why not?

b Find the frame height for Ben, whose inseam length is 75 cm.

Formulas are often written with the result at the beginning of the

formula

saddle height (in cm)  inseam (in cm)  1.08frame height (in cm)  inseam (in cm)  0.66 + 2

25 Study this bike.

a What is the frame height?

b What is the saddle height?

c Do both of these numbers correspond to the same inseam

length? How did you find your answer?

Section C: Formulas 21

54 cm

81 cm

A formula shows a procedure that can be applied over and over againfor different numbers in the same situation

Bike shops use formulas to fit bicycles to their riders

inseam (in cm)  0.66
2  frame height (in cm)

Written with the result first:

frame height (in cm)  inseam (in cm)  0.66
2Many formulas can be described with arrow strings; for example:

inseam  ⎯⎯→0.66 ⎯⎯⎯⎯→
2 frame heightYou can use the string to make a table for the formula From the data

in the table, you can draw a graph Some problems are easier to solvewith a graph, some are easier to solve using a formula, some with anarrow string, and some are easier to solve using a table You havestudied many strategies to solve problems

1 a Write the following formula about taxi costs as an arrow string.

total price = number of miles  $1.40
$1.90

b Why is it useful to write a formula as an arrow string?

C

Section C: Formulas 23

The manager of the Corner Store wants to help customers estimate

the total cost of their purchases She posts a table next to a scale

2 Help the manager by copying and completing this table.

This picture shows a stack of chairs

Notice that the height of one chair

is 80 cm and a stack of two chairs is

87 cm high

Damian suggests that the following arrow string can be used to find the height of a stack of these chairs

What numbers should Alba use in her arrow string?

Explain your answer

This graph represents the height of stacks of chairs The number ofchairs in the stack is on the horizontal axis, and the height of the stack

is on the vertical axis

4 a What does the point labeled A represent?

b Does each point on the line that is drawn have a meaning?

Explain your reasoning

c Explain why the graph will not intersect the point (0, 0).

d Use the graph to determine the number of chairs that can be

put in a stack that will fit in a storage space of 116 cm high

e Check your answer using an arrow string.

State your preferences for using a graph or an arrow string to displaythe saddle height for a bicycle Explain why you think this is the betterway to describe the data

50 100 150 200 250

Number of Chairs

A

C

Marty is going to visit Europe He wants to prepare himself to use thedifferent forms of currencies and different units of measure He knowsdistances in Europe are expressed only in kilometers and never inmiles He looks on the Internet for a way to convert miles to kilometers.The computer uses an estimate for the relationship between milesand kilometers

1 Reflect Think of a problem that Marty can do using the converterwhile he is traveling

Section D: Reverse Operations 25

Reverse Operations

Distances

Miles: Kilometers: Conversion

Enter miles or kilometers and click the “Calculate” button.