A tank is filled by one tap in 20 minutes and by another in 25 minutes. If both tapes are kept open for 5 minutes and then the second is turned off. In how many minutes the tank will be completely filled?

16 minutes

17 minutes

11 minutes

None of these

Let's calculate the work done by both taps in 5 minutes:

Tap 1 fills the tank in 20 minutes, so in 1 minute it fills 1/20 of the tank. In 5 minutes, it fills 5/20 = 1/4 of the tank.

Tap 2 fills the tank in 25 minutes, so in 1 minute it fills 1/25 of the tank. In 5 minutes, it fills 5/25 = 1/5 of the tank.

Combined work in 5 minutes = 1/4 + 1/5 = (5 + 4)/20 = 9/20

Work left = 1 - 9/20 = 11/20

Now, Tap 1 fills 1/20 of the tank in 1 minute. To fill 11/20 of the tank, it will take:

(11/20) / (1/20) = 11 minutes