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if it takes 3 people 8 hours to wash 15 cars, how long would it take 2 people to wash 7 1/2 cars?
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The rate of work calculated based on the first part of the condition, is
car-per-person-per-hour.
Let t be the time under the question. The the rate of work calculated based on this (second) part of the problem is
: (2*t) = .
We assume that the rate of work remains the same. It gives us an equation
= .
First reduce what you can:
= .
Now solve this proportion for t:
t = 3*2 = 6.
Answer. It will take 6 hours.
If you want to see more solved rate of work problems, look into the lesson Rate of work problems in this site.
At the end of that lesson you will find the list of other types rate of work problems that might be interesting to you, too.