Question 923721
Does " 12 filled " mean " 1/2 filled" ?
I'll bet it does
If so, 
The rate of filling is:
( 1 pool filled ) / ( 36 hrs )
The rate of draining is:
( 1 pool drained ) / ( 45 hrs )
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Add the rates to get their rates together
Make the draining rate negative, though
Let {{{ t }}} = time in hrs to fill 1/2 of the pool
{{{ 1/36 - 1/45 = (1/2) / t }}}
{{{ 1/36 - 1/45 = 1/(2t) }}}
Multiply both sides by {{{ 2*36*45t }}}
{{{ 90t - 72t = 36*45 }}}
{{{ 18t = 1620 }}}
{{{ t = 90 }}}
It will take 90 hrs to fill the pool
check:
{{{ 1/36 - 1/45 = 1/(2*90) }}}
{{{ 1/36 - 1/45 = 1/180 }}}
{{{ 5/180 - 4/180 = 1/180 }}}
{{{ 1/180 = 1/180 }}}
OK