A tank is filled in 13 minutes by three taps running together. Tap A is twice as fast as tap B, and tap B is twice as fast as tap C. How much time will tap A take to fill the tank?
Let the time taken by tap A be x mins. Then 13 minutes work of all the taps should add to 1. So we have, $13 × 1/x + 13 × 2/x + 13 × 4/x$ = 1, which is same as $13 × 7/x$ = 1. Solving, we get x = 91 mins.