IBPS RRB Officer Scale 1 Exam 12 Sep 2020 Prelims Shift 1 Paper

Three pipes A, B and C were opened to fill a tank. Working alone A, B and C requires 12 minutes, 15 minutes and 20 minutes respectively. Another pipe D, which is outlet pipe, can empty the fully filled tank in 30 minutes working alone. What is the total time taken to fill the tank if all the pipes are simultaneously opened?