Question 820898
Together, the rate of fil;ling is:
( 1 tank filled ) / ( 3 hrs )
The rate of filling of the smaller pipe is:
( 1 tank filled ) / ( 7 hrs )
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Let {{{ t }}} = the time in hrs for the larger pipe 
to fill the tank
Add the rates of filling of the 2 pipes:
{{{ 1/t + 1/7 = 1/3 }}}
Multiply both sides by {{{ 21t }}}
{{{ 21 + 3t = 7t }}}
{{{ 4t = 21 }}}
{{{ t = 5.25 }}} hrs
{{{ .25*60 = 15 }}} min
It will take the larger pipe 5 hrs and 15 min
to fill the tank