Two pipes A and B can fill a tank in 15 hours and 20 hours respectively while a third pipe C can empty the full tank in 25 hours. All the three pipes are opened in the beginning. After 10 hours C is closed. In how much time will the tank be full? `12\ h r s` b. `13\ h r s` c. `16\ h r s` d. `18\ h r s`