A tap that could fill it in 15 minutes will fill it 1/15 per minute. Likewise a tap that would fill it in 20 minutes has a rate of 1/20 per minute. Then you have a waste pipe that would empty it in x minutes, which subtracts 1/x per minute. Then in 30 minutes 1/30 = 1/15 + 1/20 -1/x or 1=2 + 1 1 ...