Lesson 27
Solving Problems with Compound
Units
Getting the idea
• We discussed rate in an earlier chapter, but
lets revisit it.
• A rate is a ratio that compares 2 kinds of units.
5 pounds per square inch is a rate that
compares two units- pounds and square
inches.
• If the denominator of a ratio is 1, it is called a
unit rate
Example 1
• On Earth, the air pressure at sea level is 14.7
pounds per square inch. The air pressure on
Venus is about 90 times that of Earth. What is
the approximate air pressure on Venus, in
pounds per square inch?
– Multiply to find the air pressure of Venus.
• The air pressure on Venus is approximately
1,323 pounds per square inch.
Example 2
• Brand A Tomato Soup costs $2.38 for 14
ounces. Brand B costs $2.28 for 12 ounces.
Which brand is the better buy?
– Choose the brand with the lower unit price.
– What is the unit price for Brand A?
• $.17 per oz
– What is the unit price for Brand B?
• $.19 per oz
– Compare! Which is the better buy? Brand A
D=rt
• Some problems can be solved using the
distance formula: d=rt where d=distance,
r=rate and t=time.
• This is NOT on your reference sheet for the
NJAsk, just FYI.
So now that we know that…
• On a trip, Delroy drives an average speed of 52
miles per hour. How far can he travel in 4
hours?
– Use d=rt.
• Identify d,r and t. r= 52 and t=4.
– Substitute the values into the distance formula
and solve.
• At that speed he can drive 208 miles in 4
hours.
Example 4
• In the year 2000, the population of Paterson,
NJ was about 149,000. Its land area (the area
of Paterson that is not water) is 8.4 miles. To
the nearest 100, find the population density,
in people per square mile, of Paterson in the
year 2000.
– What do we do to solve this problem?
– What am I trying to find?
Strategy
• Divide the population by the area.
• If there are 149,000 people in 8.4 square
miles, then…..
– There were 17,700 people per square mile in
Paterson in the year 2000.
On your own
• Numbers 1-8 on pg 164-165
• We will do the open ended together.