Question 692396
1st pipe alone can fill the tank in 27 mins.
So in 1 minute, it can fill 1/27 part of the tank.
2nd pipe alone can fill the tank in 54 mins.
So in 1 minute, it can fill 1/54 part of the tank.
3rd pipe can empty the tank in 36 mins.
So in 1 minute, it can empty 1/36 part of the tank.
When all pipes are open, in 1 minute tank fills
{{{1/27 + 1/54 - 1/36}}}
= {{{1/36}}} parts (do the calculation yourself)
So, they will fill the tank in 1 / (1/36) mins = 36 mins.