<PRE><B>Question 338726</B>
 If it takes 15 minutes to fill a pool and 20 minutes to drain it, how long would it take to fill the pool if the drain is left open?
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Fill rate = 1/15 pool/min
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Drain rate = 1/20 pool/min
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Together rate = 1/x pool/min
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1/15 - 1/20 = 1/x
Multiply thru by 60x to get:
4x - 3x = 60
x = 60 minutes to fill the pool
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Cheers,
Stan H.</PRE>