Question 1107747
The rate of filling is [ 1 tank ] / [ 8 hrs ]
The rate of emptying is [ minus 1 tank ] / [ 16 hrs ] 
( twice as long and removing water )
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Let {{{ t }}} = time in hrs to fill the tank with both pipes open
{{{ 1/8 - 1/16 = 1/t }}}
Multiply both sides by {{{ 16t }}}
{{{ 2t - t = 16 }}}
{{{ t = 16 }}}
It will take 16 hrs to fill the tank with both pipes open