There are three pipes connected with a tank. The first pipe can fill 12 part of the tank in 1 h, second pipe can fill 13 part of the tank in 1 h. Third pipe is connected to empty the tank. After opening all the three pipes, 712 part of the tank can be filled in 1 h, then how long will third pipe take to empty the full tank?

Correct Answer: 4 h

1st pipe takes 1 h to fill 1/2 part of the tank.

So, time taken to fill the whole tank (m) = 2 h

2nd pipe takes 1 h to 1/3 part of the tank

So. time taken to fill the whole tank (n) = 3 h

Let 3rd pipe takes P h to empty the tank = x

Therefore, 1/m+ 1/n- 1/x= 7/12

= 1/2+ 1/3- 1/x = 7/12

= 1/x=6+4−7/12=3/12=1/4

Therefore, x = 4 h