Question 677555
Add the rates of pumping to get the rate 
pumping together
( blood pumped by small pump ) / ( pumping time ) + ( blood pumped by large pump ) / ( pumping time )
= ( blood pumped with both pumps ) / ( pumping time )
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{{{ 1/30 + 1/20 = 1/t }}}
Multiply both sides by {{{ 60t }}}
{{{ 2t + 3t = 60 }}}
{{{ 5t = 60 }}}
{{{ t = 12 }}}
It will take 12 minutes with both pumps hooked up