Tap P can fill a cistern in 6 hours and tap Q can empty the full cistern in 10 hours. If both taps P and Q are kept open simultaneously, then in how many hours will the empty cistern be completely full?

15 18 16 12

15

P+ = 6 hrs, Q- = 10 hrs, ⇒ Time required for P + Q to completely fill the tank alone = \(\frac{30}{5-3}\) = \(\frac{30}{2}\) = 15 hrs.