Pipe D1 can fill an empty tank in 80 minutes and Pipe D2 can empty the same tank in 100 minutes. If pipes are opened simultaneously, then in how much time the tank gets completely filled ?

400 minutes

D1+ = 80 min, D2- = 100 min,

⇒ Time required by D1 + D2 to fill the tank completely = \(\frac{400}{5-4}\) = \(\frac{400}{1}\) = 400 minutes.