A tank is filled in 15 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 15 minutes work of all the taps should add to 1. So we have, $15 × 1/x + 15 × 2/x + 15 × 4/x$ = 1, which is same as $15 × 7/x$ = 1. Solving, we get x = 105 mins.