Question 570697
Let the time to drain the pool for 2nd pump = {{{t}}}
The 1st pump's rate of draining is {{{ 1/9 }}} 
which means ( 1 pool ) / ( 9 min )
The 2nd pump's rate of draining is {{{ 1/t }}}
which means ( 1 pool ) / ( t minutes )
Together, their rate of draining is {{{ 1/5 }}}
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To find the rate when both are used, add their
individual rates
{{{ 1/9 + 1/t = 1/5 }}}
Multiply both sides by {{{ 45t }}}
{{{ 5t + 45 = 9t }}}
{{{ 4t = 45 }}}
{{{ t = 11.25 }}}
{{{ .25*60 = 15 }}}
The 2nd pump can drain the pool in 11 min and 15 sec
operating alone