Pipes A and B can fill a tank in 12 hours and 16 hours respectively and pipe C can empty the full tank in 24 hours. All three pipes are opened together, but after 4 hours pipe B is closed. In how many hours, the empty tank will be completely filled?

18

A+ = 12 hrs, B+ = 16 hrs, C- = 24 hrs,

⇒ A + B + C opened for 4 hrs and the B was closed = (4 + 3 - 2) x 4 = 5 x 4 = 20 units,

⇒ Remaining = 48 - 20 = 28 units,

⇒ Time required for A + C to fill remaining cistern = $\frac{28}{4-2}$ = $\frac{28}{2}$ = 14 hrs,

⇒ Total time taken to fill the cistern = 4 + 14 = 18 hrs.