3 pumps, working 8 hours a day, can empty a tank in 2 days. How many hours a day must 4 pumps work to empty the tank in 1 day?

Explanation

The total work equals 3 pumps × 8 h/day × 2 days = 48 pump-hours. With four pumps working for one day, 4h = 48, so h = 12.