Two pipes P and Q can fill an empty tank in 20 min and 10 min respectively. R can empty a full tank in 15 minutes. If all three pipes are opened together, how much time (in minutes) will they take to fill the tank?

12

P+ = 20 min, Q+ = 10 min, R- = 15 min,

⇒ Time required by P + Q + R to completely fill the tank = \(\frac{60}{3+6-4}\) = \(\frac{60}{5}\) = 12 min.