SOL 7.4, 8.3a Use measures expressed as rates (e.g., speed, density) and measures expressed as products (e.g., person-days) to solve problems; check the units of the solutions; and use dimensional analysis to check the reasonableness of the answer.
Objective: Finding Unit Rates Finding Unit Prices to Compare Costs
Rate Movie
Ratio: 903
Rate: 90 miles3 hours
Read as “90 miles per 3 hours.”
A rate is a comparison of two quantities measured in different units.
Notes
Unit rates are rates in which the second quantity is 1.
unit rate: 30 miles,1 hour
or 30 mi/h
The ratio 903
can be simplified by dividing:
903
= 301
Notes
Check It Out! Example 1
Penelope can type 90 words in 2 minutes. How many words can she type in 1 minute?
90 words 2 minutes
Write a rate.
=
Penelope can type 45 words in one minute.
90 words ÷ 2 2 minutes ÷ 2
Divide to find words per minute.
45 words 1 minute
Estimate each unit rate.
Additional Example 3A: Estimating Unit Rates
Choose a number close to 468 that is divisible by 91.
468 students to 91 computers
468 students to 91 computers is approximately 5 students per computer.
468 students91 computers
455 students91 computers
5 students1 computer
Divide to find students per computer.
Estimate each unit rate.
Check It Out! Example 3A
Choose a number close to 583 that is divisible by 85.
583 soccer players to 85 soccer balls.
583 soccer players to 85 soccer balls is approximately 7 players per soccer ball.
583 players85 soccer balls
595 players85 soccer balls
7 players1 soccer ball
Divide to find players per soccer ball.
Pactice
• Estimate each unit rate– 121 students in 3 buses
– 31.50 for 4 hours
Unit price is a unit rate used to compare price per item.
Pens can be purchased in a 5-pack for $1.95 or a 15-pack for $6.20. Which pack has the lower unit price?
Additional Example 4A: Finding Unit Prices to Compare Costs
Divide the price by the number of pens.
price for packagenumber of pens
=$1.955
= $0.39
price for packagenumber of pens
= $6.2015
$0.41
The 5-pack for $1.95 has the lower unit price.
Check It Out! Example 4B
John can buy a 24 oz bottle of ketchup for $2.19 or a 36 oz bottle for $3.79. Which bottle has the lower unit price?
$2.1924
= $0.09
= $3.7936
$0.11
The 24 oz jar for $2.19 has the lower unit price.
price for bottlenumber of ounces
price for bottlenumber of ounces
Divide the price by the number of ounces.
Lesson Quiz: Part I
1. Meka can make 6 bracelets per half hour. How many bracelets can she make per hour?
2. A penny has a mass of 2.5 g and a volume of approximately 0.360 cm3. What is the approximate density of a penny?
Estimate each unit rate.
3. $2.22 for 6 stamps
4. 8 heartbeats in 6 seconds
$0.37 per stamp
≈ 6.94 g/cm3
1.3 beats/s
12
Lesson Quiz: Part II
Find each unit price. Then tell which has the
lower unit price.
5. A half dozen carnations for $4.75 or a dozen
for $9.24
6. 4 pens for $5.16 or a ten-pack for $12.90.
a dozen
They cost the same.
