Two taps A and B can fill a tank in 16 and 32 minutes respectively. Both the taps are turned on at the same time. After how many minutes should B be turned off so that the tank can be filled in 12 minutes?
Let B be closed after x mins. Then sum of works done by A and B = 1. $12/16 + x/32 = 1$. Solving, we get x = 8.