Question 437413
Let {{{t}}} = time for larger pipe to fill oil tank
given:
{{{ t + 6 }}} = time for smaller pipe to fill tank
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Add their rates of filling to get rate working together
{{{ 1/t + 1/(t+6) = 1/4 }}}
Multiply both sides by {{{ 4t*(t+6) }}}
{{{ 4*(t + 6) + 4t = t*(t+6) }}}
{{{4t + 24 + 4t = t^2 + 6t }}}
{{{ t^2 - 2t - 24 = 0 }}}
{{{ (t-6)*(t+4) = 0 }}}
The solutions are 
{{{t = -4}}} (can't use negative time)
{{{t = 6}}}
and
{{{ t + 6 = 12}}}
The smaller pipe takes 12 min 
The larger pipe takes 6 min