Question 1020668
Reciprocal of the sum of the individual rates.


<pre>
PIPE         RATE

inlet        {{{0+1/4}}}

hose         {{{0+1/7}}}

outlet        {{{-1/9}}}
</pre>

All pipes working simultaneously, the rate in POOLS per HOUR is {{{1/4+1/7-1/9}}}.


First step is  {{{(1/4)(63/63)+(1/7)(36/36)-(1/9)(28/28)}}}.


Next step can be {{{(63+36-28)/(4*7*9)}}}.