Question 640140
When a drain is closed, a swimming pool can be filled in 4 hours. When the drain is opened it takes 5 hours to empty the pool. The pool is being filled, but the drain was accidently left open. How long until the pool is filled?
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let x=time it takes to fill the pool
fill rate=1/4
drain rate=1/5
..
x/4-x/5=1
5x-4x=20
x=20
time it takes to fill the pool=20 hrs