Question 714263
Let {{{ r }}} = the rate of filling for smaller pipe
{{{ 2r }}} = the rate of filling for larger pipe
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With both pipes open:
{{{ r + 2r = 1/8 }}}
( note that 1/8 means  1 tank filled  /  8 min time )
{{{ 3r = 1/8 }}}
{{{ r = 1/24 }}}
{{{ 2r = 1/12 }}}
Using the smaller pipe alone, it takes 24 hrs to fill the tank