WBCS.LIVE practice question

Two pipes, working together, can fill a tank in 3 hours, and 4 hours respectively while a third pipe can empty the tank in 8 hours. When the tank was full, all three pipes were opened together. How much time will it take to fill the $\frac{11}{12}\mathrm{th}$ part of the tank?

Solve this multiple-choice question and check the correct answer with its explanation below.