Question 134620
Think about what happens in the time it takes to 'fill the tank'. With the drain open, 6/8th of the water will drain in the time it would take to fill the tank with the drain closed.

So instead of having a full tank, we have a a tank that is (1 - 3/4) full = 1/4 full.

So the tank fills 1/4 of the way every 6 minutes. 

How many minutes to fill it at that rate?? We will need to add in 1/4 + 1/4 + 1/4 + 1/4 = 1 

We need 4 times the 6 minutes to fill the tank. ==> 24 minutes


Look at it another way, You could fill the tank 4 times in 24 minutes if the drain was closed. You could drain the tank 3 times in 24 minutes if you had 3 full tanks ready to rock and roll.

Net 4 - 3 = 1 tank after 24 minutes.