<p>It looks like this last formula is easily derived from substituting the average rate (from Xiggi’s formula) into the usual formula d=rt as follows: </p>
<p>d=rt = [2(r1r2)/(r1+r2)]t</p>
<p>Since this is the distance for both parts of the trip, if we get rid of the 2, we get the distance for just the first (or second) part of the trip.</p>