Question 1026400
<pre>                rate       time       distance
upstream         r-c        7          21
downstream       r+c        3          21</pre>

{{{system(7r-7c=21,3r+3c=21)}}}


Most convenient is to use Elimination Method to solve for c.
{{{system(21r-21c=21*3,21r+21c=21*7)}}}
{{{42c=21*7-21*3}}}
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