12 hours will it take to fill the given pool .
we can solve this problem using unitary method ,
10 hours is required to fill the full pool
1 hour is required to fill 1/10th of the pool
30 hours is required to empty the pool
1 hour is required to empty 1/30th of the pool
If two pipes are open then in 1 hour ( 1/10 - 1/30 = 2/30 = 1/15 ) pool will be filled .
The pool is 1/5th filled already as it is mentioned .
So , remaining is ( 1 - 1/5 = 4/5 )
If 1/15th is filled in 1 hour , then 4/5th will be filled in ( 15 × 4/5 ) hours = 12 hours
So, it will take 12 hors to fill the pool .
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