Question 496833
After 20 min, there is {{{ 9000 - 30 = 8970 }}} gallons
left.
At {{{ t = 0 }}}, there was {{{ 9000 }}} gallons in the pool
If gallons remaining is {{{v}}}, and
time of pumping is {{{t}}}, the the point-slope formula is:
{{{ ( v - 8970 ) / ( t - 20 ) = ( 9000 - 8970 ) / ( 0 - 20 ) }}}
{{{ ( v - 8970 ) / ( t - 20 ) = 30 / -20 }}}
Multiply both sides by {{{ -20*( t - 20 ) }}}
{{{ -20*( v - 8970 ) = 30*( t - 20 ) }}}
{{{ -20v + 179400 = 30t - 600 }}}
{{{ -20v = 30t - 180000 }}}
{{{ v = -(3/2)*t + 9000 }}}
check answer:
{{{ v = 8970 }}}
{{{ t = 20 }}} 
{{{ 8970 = -(3/2)*20 + 9000 }}}
{{{ 8970 = -30 + 9000 }}}
{{{ 8970 = 8970 }}}
OK