One inlet pipe can fill an empty pool in 8 hours, and a drain can empty the pool in 20 hours. How long will it take the pipe to fill the pool if the drain is left open?

Answer 1

The fill rate is (1/8) pool per hour

the drain rate is (1/20) pool per hour

Simultaneous rate is 1/8 - 1/20 = 5/40 - 2/40 = 3/40 pool per hour

So it will take 40/3 = 13 and 1/3 hours = 13 hours and 20 minutes