Question 1192916
<pre>
Make this chart. Let r = rate in still air, and
let w = the rate of the wind.

              | Rate | Time | Distance
-----------------------------------------         
With the wind | r+w  |   4  |  1680
-----------------------------------------
Against wind  | r-w  |   5  |  1680 


               Rate  x Time = Distance 

                   (r+w)(4) = 1680 
                   (r-w)(5) = 1680

                {{{system((r+w)(4)=1680,(r-w)(5)=1680)}}}

                Divide the first through by 4 and the
                second through by 5:

                {{{system(r+w=420,r-w=336)}}} 

Add those two equations term by term, and the w's will cancel
and you can find r.  Then substitute to find w.

Edwin</pre>