Question 1057931
The sum of the individual rates is the rate for both pipes combined.  The rate unit should be as {{{TANKS/HOUR}}}.

<pre>
PIPE                RATE
SMALL              {{{1/(x+1.7)}}}
LARGE              {{{1/x}}}
BOTH TOGETHER      {{{1/8.4}}}
</pre>
{{{1/(x+1.7)+1/x=1/8.4}}}