(15 min) Rates
Rates involve numbers and units expressed as fractions. They are
expressed with a "/" or "____" which is read as "per". Rates usually
measure speed, with time as the denominator, but sometimes there are other
units instead.
Sally types 45 words 45 words/min 45 words
per minute -----
min
I drove 30 miles 30 miles/hr 30 miles
per hour -----
hour
Pistachios cost five $5/lb. $5
dollars per pound --
lb
Both numbers and units can be multiplied and canceled when using rates.
(All problems in this section assume a constant rate.)
I drove 30 miles
per hour. How far did I drive in 4 hours?
30 miles x 4 hours = 120 miles
-----
hour
I drove 30 miles per hour. How fast did I drive per minute?
30 miles x 1 hour = 1 mile
----- ---------- - -------
hour 60 minutes 2 minutes
I drove 1/2 mile in 1 minute! You know you have it set up right if the
hours cancel. This is wrong:
30 miles x 60 minutes 180 mile-minutes
----- ---------- = ------------
hour 1 hour hour2