Question

Four employees can wash 20 service vehicles in 5 hours. how long would it tke 5 employees to wash the same number of vehicles?

Answer 1

So the employee rate for washing cars turns out to be 4*5 = 20 man hours to wash 20 vehicles which is 1 vehicle per hour per employee. 
5 employees will wash 5 vehicles in 1 hour 
5 employees will wash 20 vehicles in 1*4 = 4 hours.