IBPS RRB PO Prelims 21 Aug 2022 Shift 2 Paper

There is a leak in the bottom of the tank. This leak can empty the tank in 15 hours. Tap A alone can fill the tank in 6 hours. Tap A opens for 4 hours after that remaining part of the tank is filled by tap B in 18 hours. When both pipes are opened simultaneously, then in what time tank is completely filled?