Notes
12 and 13 Be sure to discuss these problems in class focusing on the strategies that are used.
13b Have students share with the class their reasoning for the model they chose.
14 You may want to do this problem together as a class. Students should recognize that there is not enough information to solve the problem.
Models You Can Count On D
Gary and Sharon like to hike. This weekend they plan to walk a 434-mi lake trail. They estimate how long they will hike. Gary uses a double number line like the one on page 43.
12. Draw a double number line and use it to find the time needed for the hike.
Sharon uses a ratio table to make the same calculation.
13. a. Explain how Sharon decided on the numbers in each new column in the table.
b. Which model do you prefer, the double number line or the ratio table? Explain your preference.
The Double Number Line
D
Section D: The Double Number Line 44T
Solutions and Samples
12. The hike will take about 95 minutes, or one hour and 35 minutes.
13. a. Answers may vary. Sample answer:
Sharon used the information from Student Book page 43 that it takes Gary 10 minutes to walk half a mile. She put the information in the first column. She then doubled the numbers in the first column. She multiplied numbers in the second column by four. She divided the numbers in the first column by 2 to find the numbers in the fourth column. She then added the numbers from the first and fourth columns, and finally she added the numbers from the third and fifth columns to find 95 in the top row.
b. Answers will vary. It is important that students give reasons for their preference of either the ratio table or the double number line. Ask students to explain what they mean when they say “It is easier to use.” Sample student work:
• For figuring out the difference from one place to another, I like the double number line because it lays it out better.
• I think the double number line model would be much more preferable because it is a lot more organized, so you don't get confused.
• I like the ratio table because it is quicker to figure the problem out.
• I like ratio tables more because they're more clear and direct.
14. Answers may vary. Sample answers:
• She will multiply the price per kilogram by 0.2 to find how much the piece of salami will be.
So Susan needs to know the price per kilogram.
• She will multiply the price of 100 g (one tenth of a kilogram) by 2, so she needs to know the price of 100 g.
Hints and Comments
Overview
Students solve another problem about miles and minutes using a double number line. They see how the same problem can be solved using a ratio table.
They explain which of these two models they prefer.
Then students start to work with decimals in the context of weights and prices.
Comments About the Solutions
12. Observe how students use the double number line. Especially see what steps they make to arrive at a solution. Do they write many numbers at the marks? Or do they need just a few steps? This may reveal how well they have developed their number sense, especially for fractions.
0 0 5 10
1 20
2 40
3 4
80
4 90
4 95
miles minutes
1 4
1 2
1 2
3 4
41.MYCCO.SecD.0629.qxd 06/30/2005 02:49 Page 49
Notes
15bPoint out that the picture shows that brie is
$12.00/kg.
18 To discuss this problem, invite some students to present one of their choices and solutions to class.
Models You Can Count On D
Ahmed is shopping for some brie, a kind of French cheese. He wants the piece to weigh about 14kg. Susan cuts off a piece and puts it on the scale.
The scale shows:
15. a. Does Ahmed have the amount of brie he wants?
b. Calculate how much Ahmed has to pay for this piece of brie.
Ahmed often buys brie at Jack’s Delicatessen. Lately he has been thinking about a clever way to estimate the price. The scale reminds him of a double number line. He creates the following double number line including both weight and price on it.
16. Show how Ahmed can use this double number line to estimate the price for 0.7 kg of brie.
This week, brie is on sale for $9.00 per kilogram.
17. What is the sale price of 0.7 kg of brie? Show how you found your answer.
18. a. Select three different pieces of brie to purchase and write down the weight of each piece.
b. Draw a scale pointer to mark each weight on a different number line.
(You may want to exchange your notebook with a classmate after both of you have drawn your pointers.) c. Estimate the regular price and the
sale price of the three pieces of brie.
The Double Number Line D
0 0.5 1 kg
0
0 0.5 1 Kg
$12 price
weight
Reaching All Learners
Intervention
If students struggle with problem 18, you can select three easy weights for them to work with; for example, 5, 1, 3.
For problem 16, students can find the amount of .1 kg ($1.20) and .5 kg ($6.00) and then see if they can find 0.7 on their own.
English Language Learners
You may need to read this page to English language learners to make sure they understand the context.
Assessment Pyramid
15ab, 17
Use a double number line to solve problems.
45 Models You Can Count On
41.MYCCO.SecD.0629.qxd 06/30/2005 02:49 Page 50
Section D: The Double Number Line 45T
Solutions and Samples
15. a. Students’ answers may vary. Sample answer:
The arrow on the scale is almost in the middle between 0 and 0.5, so it is at 0.25, or __14 . Ahmed gets the weight of brie he wants.
b. __14 of $12 equals $3; Ahmed has to pay $3.
16. Solutions may vary. Sample students’ solutions:
Ahmed may estimate the price somewhere between $8 and $9.
Some students may write numbers with all the vertical marks. In fact, they do not estimate.
17. The price is now $6.30.
Sample solution:
18. a.– c.
Students’ answers will vary. Sample student solutions:
0.3 kg; 0.1 kg; 0.9 kg
Hints and Comments
Overview
Students estimate and calculate the price of different weights of brie. They start to use a double number line with price and weight, to make their estimations.
About the Mathematics
In this context, students start informally to multiply fractions and decimals with whole numbers. They review numbers with one decimal from Section C, but now the units are kilograms instead of miles. On this page, the prices per kilogram are whole numbers of dollars; on the next page the prices per kilogram are decimals.
Students will learn more strategies for multiplying whole numbers with decimals in the unit Fraction Times.
Comments About the Solutions
16. Depending on what you observed in class, you may want to discuss the difference between making an estimate and making an accurate calculation. You may use the solutions in the Solutions and Samples for this purpose or the work from your own students.
17. Student strategies will show whether they choose to use a double number line. When a student did not use a double number line and did not solve the problem correctly, you may ask why he or she did not use a double number line. Then ask the student to redo the problem using a double number line and observe whether this is helpful.
Weight (kg) Regular Price ($) Sales Price ($) 0.3 $3.60 or $4.00 $2.70
0.1 $1.20 $0.90 or $1.00
0.9 $11.00 $8.00
0
0 0.5 0.7
$6 $9
1 kg
$12 price
weight
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
$1.20 $3.60 $2.40 $4.80 $7.20 $9.60 $6
$8.40
1 kg
$12 price
weight
0
0 0.1 0.5 0.7
$0.90 $4.50 $5.40 $6.30
1 kg
$9 price
weight
0
0 0.1 0.3 0.5 0.9 1 kg
$9
$12
price
weight 41.MYCCO.TG.SecD.0914.qxd 11/19/2005 17:07 Page O
Notes
19 Point out that students could round $1.89 to $1.90.
This will make the numbers easier to work with.
19 –21 Discuss students’
solutions in class, focusing the discussion on how they made their estimates.
Models You Can Count On D
Jack’s Delicatessen also sells fruit. This week, California grapes are on sale for $1.89 per kilogram.
Madeleine places her grapes on the scale.
This is what she sees.
19. a. How much do Madeleine’s grapes weigh?
b. Estimate how much Madeleine has to pay for the grapes. You may want to use a double number line for your estimation and explanation.
Ahmed decides to use the money he has left to buy some grapes. He counts his money and realizes he has $1.25.
20. Estimate what weight of grapes Ahmed can buy for $1.25. Show how you found your answer.
Susan weighed fruit for 5 customers. She wrote the prices on small price tags, but, unfortunately, the tags got mixed up.
21. Match the information written below to the corresponding price tags. Find out which note belongs to each customer.
Show your work.
Mounim 0.5 kg grapes for $1.50/kg Claire 1.8 kg apples for $1.25/kg Frank 2.5 kg oranges for $1.90/kg Nadine 1.1 kg bananas for $2.50/kg Gail 0.9 kg kiwis for $2.40/kg
The Double Number Line
D
0 0.5 1 2kg
Grapes $1.89/kg
$4.75 $0.75 $2.75
$2.25 $2.16
Reaching All Learners
Accommodation
Make tags and names for problem 21. Students can then match the tags to the names rather than drawing lines. This problem can also be made into a game.
46 Models You Can Count On
41.MYCCO.SecD.0629.qxd 06/30/2005 02:49 Page 52
Section D: The Double Number Line 46T
Hints and Comments
Materials
transparency of the double number line from problem 19, optional (several per class)
Overview
Students read the weight of grapes and estimate the cost. They estimate what weight a person can buy for a given amount of money. Then they match price tags with different weights of fruit, priced with different amounts of money per kilogram.
About the Mathematics
Students’ implicit knowledge of decimals is expanded.
Most students are able to work with decimals within the context of money.
It is important students have a few “reference” points when estimating. For example:
Whole numbers: $1.89 is a little less than $2.00.
Halves: 1.4 kg is about 1.5 kg.
Quarters: $1.89 is a little more than $1.75.
Since working with whole numbers is often easier than working with decimals, students can convert dollar amounts to cents to simplify computation.
Planning
Students can work on problems 19–21 in small groups.
Comments About the Solutions
19. b. It may be helpful to make a transparency of the double number line so that students can show their solutions and strategies on the overhead.
21. You may encourage students to convert dollar amounts into cents. Ask students to keep track of which tag they matched first, second, and so on. This information can be used in the class discussion. Have students explain how they got their answers.
Solutions and Samples
19. a. Madeleine’s grapes weigh 1.4 kilograms.
b. Estimates and explanations will vary. Sample answers:
• You can round $1.89 to $1.90. Madeleine has to pay $1.90 (4 $0.19), which is about
$1.90 (4 $0.20) $1.90 $0.80 $2.70.
• $1.89 is about $2.00, so 1.4 kg will cost about 1.4 $2 $2.80.
20. Ahmed can buy a maximum of 0.6 kg (or 600 g) of grapes. Strategies will vary. Sample strategies:
• using the double number line making an estimation:
• using the result from problem 19: Two kilograms of grapes cost $3.80, so 0.2 kg costs $0.38. I added the same amount twice and found that 0.6 kg costs $1.14. That is the maximum amount Ahmed can spend; he has $1.25 $1.14 $0.11 left.
0.6 kg cost about 95¢ 19¢ which is about 115¢, or $1.15, or 100 cents 20 cents 120 cents.
21. Mounim $0.75 Claire $2.25 Frank $4.75 Nadine $2.75
Gail $2.16
Students’ strategies will differ; some may use number lines, while other may make calculations or estimations.
Sample calculations:
Mounim: 0.5 kg, is half a kilogram.
$1.50 divided by 2 is $0.75.
Nadine: 1.1 kg is one kilogram and one-tenth of a kilogram. The bananas costs $2.50 $0.25 $2.75.
Frank: 2.5 kg is 2 kilograms and half a kilogram;
2 kg cost 2 $1.90 $3.80; half a kilogram costs
$0.95, so $3.80 $0.95 is about $4.80.
00.1 0.5
$0.19 $1.90 $3.80
1 2kg
0 0.50.6
$1.00
$1.20 $2.00 $4.00
1 2kg
41.MYCCO.SecD.0629.qxd 06/30/2005 02:49 Page 53
Notes
Read and discuss the Math History with the class.
Models You Can Count On D
Edmund Gunter (1581-1626), an English mathematician, invented a measurement tool for surveying.
It came into common usage around 1700 and was the standard unit for measuring distances for more than 150 years.
Gunter's Chain is 66 ft long. Its usefulness comes from its connection to decimals; it is divided into 100 links.
22. How many chains make one mile?
Because Gunter's chain was used to measure America, the United States did not use the metric system (developed in France in 1790).
The Double Number Line D