Three Pipes Attached to a Tank

For Every 3 minutes volume of water filled ---  1/20 + 1/30 -1/15 = 1/60

So for 3 minutes 1 part will be filled out of 60 parts.

Total to Fill full tank is 3 * 60  = 180 mins = 3hours

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The part of tank filled by Pipe A in 1 min = 1/20
The part of tank filled by Pipe B in 1 min = 1/30
The part of tank emptied by Pipe C in 1 min = 1/15
The part of tank filled in 1 min is 1/20 + 1/30 - 1/15 = 1/60

The total time taken to fill the tank is 60 min = 1 hr.

you can not fill the tank while pipe c is still opened.. a & b is for storage and pipe C is for draining..