Lesson 8How Much for One?

Learning Goal

Let’s use ratios to describe how much things cost.

Learning Targets

  • I can choose and create diagrams to help me reason about prices.

  • I can explain what the phrase “at this rate” means, using prices as an example.

  • If I know the price of multiple things, I can find the price per thing.

Lesson Terms

  • double number line diagram
  • per
  • unit price

Warm Up: Number Talk: Remainders in Division

Problem 1

Find the quotient mentally.

Activity 1: Grocery Shopping

Answer each question and explain or show your reasoning.

Problem 1

A picture of an avocado cut in half.

Eight avocados cost $4.

Use the applet if you choose to:

open applet in presentation mode

  1. How much do 16 avocados cost?

  2. How much do 20 avocados cost?

  3. How much do 9 avocados cost?

Print Version
A picture of an avocado cut in half.

Eight avocados cost $4.

  1. How much do 16 avocados cost?

  2. How much do 20 avocados cost?

  3. How much do 9 avocados cost?

Problem 2

A picture of water bottles.

Twelve large bottles of water cost $9.

Use the applet if you choose to:

open applet in presentation mode

  1. How many bottles can you buy for $3?

  2. What is the cost per bottle of water?

  3. How much would 7 bottles of water cost?

Print Version
A picture of water bottles.

Twelve large bottles of water cost $9.

  1. How many bottles can you buy for $3?

  2. What is the cost per bottle of water?

  3. How much would 7 bottles of water cost?

Problem 3

A 10-pound sack of flour costs $8.

Use the applet if you choose to:

open applet in presentation mode

  1. How much does 40 pounds of flour cost?

  2. What is the cost per pound of flour?

Print Version

A 10-pound sack of flour costs $8.

  1. How much does 40 pounds of flour cost?

  2. What is the cost per pound of flour?

Are you ready for more?

Problem 1

It is commonly thought that buying larger packages or containers, sometimes called buying in bulk, is a great way to save money. For example, a 6-pack of soda might cost $3 while a 12-pack of the same brand costs $5.

Find 3 different cases where it is not true that buying in bulk saves money. You may use the internet or go to a local grocery store and take photographs of the cases you find. Make sure the products are the same brand. For each example that you find, give the quantity or size of each, and describe how you know that the larger size is not a better deal.

Activity 2: More Shopping

Problem 1

Four bags of chips cost $6.

open applet in presentation mode

  1. What is the cost per bag?

  2. At this rate, how much will 7 bags of chips cost?

Print Version

Four bags of chips cost $6.

  1. What is the cost per bag?

  2. At this rate, how much will 7 bags of chips cost?

Problem 2

At a used book sale, 5 books cost $15.

open applet in presentation mode

  1. What is the cost per book?

  2. At this rate, how many books can you buy for $21?

Print Version

At a used book sale, 5 books cost $15.

  1. What is the cost per book?

  2. At this rate, how many books can you buy for $21?

Problem 3

Neon bracelets cost $1 for 4.

open applet in presentation mode

  1. What is the cost per bracelet?

  2. At this rate, how much will 11 neon bracelets cost?

Print Version

Neon bracelets cost $1 for 4.

A picture of neon bracelets.

  1. What is the cost per bracelet?

  2. At this rate, how much will 11 neon bracelets cost?

Problem 4

Your teacher will assign you one of the problems. Create a visual display that shows your solution to the problem. Be prepared to share your solution with the class.

Lesson Summary

The unit price is the price of 1 thing—for example, the price of 1 ticket, 1 slice of pizza, or 1 kilogram of peaches.

If 4 movie tickets cost $28, then the unit price would be the cost per ticket. We can create a double number line to find the unit price.

"A double number line with 6 evenly spaced tick marks. The top number line is labeled "cost in dollars" and the numbers 0, 7, 14, 21, 28, and 35 are indicated. The bottom number line is labeled "number of tickets" and the numbers 0, 1, 2, 3, 4, and 5 are indicated. There is a circle around the numbers 7 and 1 and a second circle around 28 and 4."

This double number line shows that the cost for 1 ticket is $7. We can also find the unit price by dividing, , or by multiplying, .