Problem solving with a proportion (ACT)

A mechanic can install carburetors in 3 cars every 4 hours.  At that rate, how long will it take the 
mechanic to install carburetors in 5 cars?
A.)  6 hours 20 minutes
B.)  6 hours 40 minutes
C.)  7 hours 15 minutes
D.)  7 hours 40 minutes
E.)  7 hours 45 minutes

3 cars             5 cars
---------   =    ------------   {set up a proportion}
4 hours           x hours

3x = 20 {cross-multiplied}
x = 20/3 {divided both sides by 3}
x = 6  2/3 {changed to mixed number}
x = 6 hours 40 minutes {2/3 of an hour is 40 minutes}

B.)  6 hours 40 minutes

- Algebra House