Pipes A and B together can fill a tank in 16 hours, where as pipe C alone can empty the full tank in 24 hours. A and B were opened together for 10 hours and then closed. Pipe C was then opened. The tank will now be emptied by C in:

15 hours

A + B = 16 hrs, C- = 24 hrs,

⇒ A + B was opened for 10 hrs = 3 x 10 = 30 units,

⇒ Time required by C to empty 30 units of tank = \(\frac{30}{2}\) = 15 hours.