A tank can be filled by one tap 20 minute and by another 25 minute. Both the taps are kept open for 5 minute and then second is turned off. In how many minute more is the tank completely filled.
Work done by both in 5 minute = (1/20 + 1/25) = 9/20
Remaining part = (1 - 9/20) = 11/20
Now, 1/20 part is filled in 1 minute. So, 11/20 part will be filled in 11 minute.
Hence, the tank will be full in 11 min more
Option 8)