Question 1131672
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The usual way to work a problem like this is to think about the fraction of the "job" each "worker" does in 1 hour.  In this problem, the input pipe FILLS 1/6 of the pool each hour and the drain EMPTIES 1/10 of the pool each hour.<br>
So if the drain is left open when the pool is being filled, the fraction of the pool that is filled in 1 hour is 1/6 - 1/10 = 5/30-3/30 = 2/30 = 1/15.<br>
So it will take 15 hours to fill the pool if the drain is left open.