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?
[A]20 minutes
[B]25 minutes
[C]30 minutes
[D]45 minutes

45 minutes
Part of the tank filled by both taps in 5 minutes
$= 5\left ( \frac{1}{30}+\frac{1}{60} \right )$
$= 5\left ( \frac{2+1}{60} \right ) = 5\times \frac{3}{60} = \frac{1}{4}$
Remaining part $= 1-\frac{1}{4} = \frac{3}{4}$ that is fileed by second tap.
$\therefore Time\ taken\ = \frac{3}{4}\times 60 = 45\ minutes$
Hence option [D] is correct answer.