<PRE><B>Question 231075</B>
It takes 10 hours to fill a pool with water, and 20 hours to drain it. If the pool is empty and the drain is open, how long will it take to fill the pool?
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Fill rate : 1/10 job/hr
Drain rate: 1/20 job/hr
Together rate: 1/x job/hr
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Equation:
1/10 - 1/20 = 1/x
Multiply thru by 20x to get:
2x - x = 20
x = 20 (number of hours required to fill the pool)
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Cheers,
Stan H.</PRE>