Question: two pipes a and b can fill a tank in 12 and 16 minutes respectively?
Answer: 4 mins before the tank to fill => the portion of the tank to be filled by only one pipe B is 4×(7/48)=(7/12). So time taken would be (7/12)÷(1/16)=(7/12)×(16)=28/3 mins=9.33min. Ans.. Two pipes can fill a cistern in 24 minutes and 32 minutes respectively.