Question 1056547
Rate of filling:
[ 1 pool filled ] / [ 12 hrs ]
Rate of emptying:
[ 1 pool emptied ] / [ 8 hrs ]
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The rate of emptying is faster than the
rate of filling, so with both pipes open, 
the pool will not fill -it will empty.
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Add the positive rate and the negative rate
to get the rate of emptying. Let {{{ t }}} = 
the time in hrs for the pool to empty
{{{ 1/12 - 1/8 = -1/t }}}
{{{ 2/24 - 3/24 = -1/t }}}
{{{ -1/24 = -1/t }}}
{{{ t = 24 }}} hrs
The pool will empty in 24 hrs