A swimming pool can be filled by a vent in 10 hours. When the pool is filled, it can be emptied bya drain pipe in 20 hours. The pool manager mistakenly leaves the drain pipe open while trying tofill the pool. How long will it take to fill the pool?

Answer 1

Net rate of filling = 1/10 - 1/20 pool/hour = 1/20 pool/hour

Fill time = (1 pool)/(1/20 pool/h)
= 20 hours