John is trying to drain a swimming pool. He has two pumps, but he can only use one at a time. He knows that the time a pump takes to drain the pool varies inversely with the power in watts of the pump

John is trying to drain a swimming pool. He has two pumps, but he can only use one at a time. He knows that the time a pump takes to drain the pool varies inversely with the power in watts of the pump. His old pump is a 40 watt pump, and it can drain the pool in 5 hours. How long would the job take if he uses his new 100 watt pump?