How long will it take a tank to be be filled by one pipe in 9 minutes and drained by another pipe in 12 minutes?

Answer 1

We're guessing that the question is trying to ask:

If one pipe can fill the tank in 9 minutes, and another pipe

can empty the tank in 12 minutes, how long does it take the

tank to fill if both pipes are open ?

So that's the question we'll go ahead and answer.

The first pipe fills 1/9th of the tank every minute.

The second pipe empties 1/12th of the tank every minute.

When they're both open, (1/9 - 1/12) of the tank fills every minute.

(1/9) - (1/12) = (4/36) - (3/36) = 1/36

Since 1/36th of the tank fills every minute, it takes 36 minutes to fill.