<PRE><B>Question 83611</B>
Time taken to fill the pool = 9 hrs

Volume filled in 1 hr = 1/9 part of the pool

Time taken to drain = 12 hrs.

Volume drained in 1 hr = 1/12 part

When drain is left open, volume filled in 1 hr = 1/9 - 1/12

                                               = (4-3)/36

                                               = 1/36

Thus time taken to fill the pool = 36 hrs when the drain is open

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