LESSON 2: Proportional Relationships
Which is the better buy?
Value-Mart is advertising a Back-to-School sale on pencils. A pack of
30 sells for $7.97 whereas a 12-pack of the same brand cost $4.77.
Which is the better buy? How do you know?
Dr. Basta
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LESSON 2: Proportional Relationships
How much am I paying for this, anyway? Is there a “better buy” at the stores?
Two new self-service ice cream stores opened this summer. At both stores, Ice Cream Emporium and Ice
Cream Treats, ice cream is sold by the weight.
Ice Cream
Emporium
(Store A)
Weight (ozs)
3
6
10
15
Cost ($)
1.20
2.40
4.00
6.00
Ice Cream
Treats
(Store B)
Weight (ozs)
3
6
10
15
Cost ($)
1.20
2.10
3.00
4.50
Dr. Basta
2
LESSON 2: Proportional Relationships
What did we learn from the ice cream store example?
At Store A, describe what people pay for the ice cream they purchase.
How much will 8 ounces of ice cream cost?
At Store B, describe what people pay for the ice cream they purchase.
How much will 8 ounces of ice cream cost?
Dr. Basta
3
LESSON 2: Proportional Relationships
Using variables and equations: y = kx
X (weight
in ozs)
12.5
Y
(cost in $)
5
12.5
5 ÷ 12.5 = 0.4
5
12.5 × 0.4 = 5
Multiplication and division are related operations.
k = 0.4
Only at Store A can we say that “cost is proportional to weight”. Even “0 ounces” can be multiplied
by $0.40 to find that the cost of 0 ounces is $0. There is a constant, k, which can be used to find the
cost of every ice cream ordered.
Dr. Basta
4
LESSON 2: Proportional Relationships
During Jose’s physical education class today, students visited activity stations. Next to each
station was a chart depicting how many Calories (on average) would be burned by completing
the activity.
Is the number of Calories burned proportional to time? How do you know?
If Jose jumped rope for 6.5 minutes, how many calories would he expect to burn?
Dr. Basta
5
LESSON 2: Proportional Relationships
Alex spent the summer helping out at his family’s business. He was hoping to earn enough money to buy a new $220
gaming system by the end of the summer. Halfway through the summer, after working for 4 weeks, he had earned
$112. Alex wonders, “If I continue to work and earn money at this rate, will I have enough money to buy the
gaming system by the end of the summer?”
To check his assumption, he decided to make a table. He entered his total money earned at the end of week 1 and
his total money earned at the end of Week 4.
Week
Total
Earnings
0
1
$28
2
3
4
5
6
7
8
$112
Are Alex’s total earning proportional to the number of weeks he worked? How do you know?
Dr. Basta
6
Problem Set: Module 1, Topic A, Lesson 2
[1] Ms. Albero decided to make juice to serve along with the pizza at the Student Government party. The directions
said to mix 2 scoops of powdered drink mix with a half a gallon of water to make each pitcher of juice. One of
Ms. Albero’s students said she will mix 8 scoops with 2 gallons of water to get 4 pitchers. How can you use the
concept of proportion to decide whether the student is correct?
[2] John is filling a bathtub that is 18 inches deep. He notices that it takes two minutes to fill the tub with three
inches of water. He estimates it will take ten more minutes for the water to reach the top of the tub if it continues
at the same rate. Is he correct? Explain.
Student Name: ____________________________
Dr. Basta
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