IBPS RRB Office Assistant Mains 7 Oct 2018 Paper

Pipes A & B can together fill a tank in 4 hours, while pipes C & D can individually empty the tank in 10 hours and 12 hours respectively. When the tank was empty, pipes A & B were simultaneously opened. After an hour, pipe C was also opened. After the opening of pipe C, pipe D will be opened after how much time such that the tank is only half filled in 3 hours?