A pipe can fill a tank in a hours and another pipe can empty it in b (b > a) hours. If both pipes are open, in how many hours will the tank is filled?

Question: A pipe can fill a tank in a hours and another pipe can empty it in b (b > a) hours. If both pipes are open, in how many hours will the tank is filled?

Solution:
Net part filled in 1 hour

∴ Net rate = Filling rate - Emptying rate
= (1/a) - (1/b)
= (b - a)/ab

∴ The tank will be filled in = ab/(b - a) hours.