Six identical pumps can empty a tank in 14 hours. After they have run for 4 hours, 2 pumps break down. How many more hours will the remaining pumps take to empty the tank?

Explanation

1. 01

   The whole job = $6 \times 14 = 84$ pump-hours. In 4 hours, $6 \times 4 = 24$ pump-hours are done.

2. 02

   Work left = 60 pump-hours, shared by 4 pumps: $60 \div 4 = 15$ hours.