Pipes A and B can empty a full tank in 36 and 45 minutes, respectively, whereas pipe C alone can fill the tank in 15 minutes. B and C are opened together for 15 minutes, and then both are turned off and A is opened. Pipe A will empty the tank in (in minutes):

24

A- = 36 min, B- = 45 min, C+ = 15 min.

⇒ B + C opened for 15 min = (-4 + 12) x 15 = 8 x 15 = 120 units,

⇒ Time required for A to completely empty 120 units = \(\frac{120}{5}\) = 24 min.