Hello there:
You can use a simple proportion, assuming that Larry never gets tired, and always washes cars at the same rate of speed.
You know that he washes
50 cars PER 20 minutes, which in math would be written
50
20
and you can compare that ratio using a proportion to the number of cars you are interested in:
80 cars PER how many??? minutes.
The whole thing (if we use t for the unknown) would look like this:
50 = 80
20 x
Now solve by using cross products:
(step one) 50 x = 20 * 80
(step 2) 50 x = 1600
(step 3) 50 x = 1600
50 50
(step 4) x = 32
Answer is 32 minutes.
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