One inlet pipe can fill an empty pool in 2 hours, and a drain can empty the pool in 4 hours. How long will it take the pipe to fill a pool if the drain is left open

Answer 1

1/2 - 1/4 = 1/p

Once we apply the common denominator, 8p, we don't need it anymore.

4p - 2p = 8

2p = 8

p = 4 hours