Q. A water tank can be filled by a tap in 30 minutes and another tap can fill it in 60 minutes. If both the taps are kept open for 5 minutes and then the first tap is closed, how long will it take for the tank to be full?
Answer: 45 minutes
Notes: Part of the tank filled by both taps in 5 minutes $latex = 5\left ( \frac{1}{30}+\frac{1}{60} \right )&s=1$ $latex = 5\left ( \frac{2+1}{60} \right ) = 5\times \frac{3}{60} = \frac{1}{4}&s=1$ Remaining part $latex = 1-\frac{1}{4} = \frac{3}{4}&s=1$ that is fileed by second tap. $latex \therefore Time\ taken\ = \frac{3}{4}\times 60 = 45\ minutes&s=1$ Hence option [D] is correct answer.

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