Inlet P is four times faster than inlet Q to fill a tank. If P alone can fill it in 15 min, how long will it take if both the pipes are opened together?

Since inlet P is four times faster than inlet Q and P alone fills the tank in 15 minutes, Q alone fills it in 60 minutes. Using the formula for combined work, their one minute work is (1/15) + (1/60) = (4/60) + (1/60) = 5/60 = 1/12. The reciprocal of 1/12 is 12, so both pipes together will fill the tank in 12 minutes.

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