Question 927307
Let {{{ R }}} = the rate of emptying of
the slowest pipe in tankfuls / hrs
{{{ 3R }}} = the rate of emptying of
the other pipe in tankfuls / hrs
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Add the rates of emptying to get
their rate emptying together
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{{{ 1/6 }}} = rate of emptying together 
in tankfuls / hrs
{{{ R + 3R = 1/6 }}}
{{{ 4R = 1/6 }}}
{{{ R = 1/24 }}}
{{{ 3R = 1/8 }}}
The faster tank takes 8 hrs to empty 
the tank alone
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check:
{{{ 1/24 + 1/8 = 1/6 }}}
{{{ 1/24 + 3/24 = 4/24 }}}
OK
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Note that if there were both emptying and
filling pipes, then the emptying rates
would have minus signs and the filling
rates would have plus signs