When the drain is closed, a swimming pool can be filled in 4 hours. When the drain is opened, it tak

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When the 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 accidentally left open. How long until the pool is completely filled?

Set up unit fill rates per hour:

• 1/4 of the pool is filled each hour
• 1/5 of the pool is drained away each hour

The amount left over after an hour of filling minus draining is:
1/4 - 1/5 = 1/20

Therefore, it take 20 hours to fill the pool