sloveing word problem

it takes 40 minutes to fill a water tank, and 60 minutes to drain it.
how many hours will it take to fill the tank while draining it the entire time ??
注水的速率為 1/40
放水的速率為 1/60
注水及同時放水則：1/40 – 1/60
= (3-2)/120
= 1/120
所以需用
1 / (1/120) = 120分鐘。