Question 816156
Add their individual rates of filling
to get their rate filling together
Let {{{ t }}} = time in minutes for
the larger pipe to fill the tank
{{{ t + 25 }}} = time in minutes for
the smaller pipe to fill tank
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( 1 tank filled / t min ) + ( 1 tank filled ) / t+25 min ) = 
( 1 tank filled / 15 min )
{{{ 1/t + 1/( t+25 ) = 1/15 }}}
Multiply both sides by {{{ 15*t*( t+25 ) }}}
{{{ 15*( t+25 ) + 15t = t*( t+25 ) }}}
{{{ 15t + 15*25 + 15t = t^2 + 25t }}}
{{{ 30t + 375 = t^2 + 25t }}}
{{{ t^2 - 5t - 375 = 0 }}}
{{{x = (-b +- sqrt( b^2-4*a*c ))/(2*a) }}} 
{{{ a = 1 }}}
{{{ b = -5 }}}
{{{ c = -375 }}}
{{{x = (-(-5) +- sqrt( (-5)^2-4*1*(-375) ))/(2*1) }}} 
{{{x = ( 5 +- sqrt( 25 + 1500 )) / 2 }}} 
{{{x = ( 5 +- sqrt( 1525 )) / 2 }}} 
You can finish- I don't have calculator