Inlet pipe:
4 hours = 1 pool
1 hour = 1/4 hoor
Drain:
10 hours = 1 pool
1 hour = 1/10 pool
If both are turned on,
1 hour = 1/4 - 1/10 = 3/20 of the pool.
Number of hours needed to fill the pool if the drain is left open:
1 ÷ 3/20
= 1 x 20/3
= 20/3
= 6 2/3 hours
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Answer: It will take 6 2/3 hours to fill up the pool.
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