Question 1011809
Solved in generalized manner because the type of question is very frequent.

KNOWN VARIABLES:  k, t, d
UNKNOWN:  r
<pre>
                   rate        time         distance
firstplane         r-k         t
secondplane        r           t
Total                                        d

                  rate        time         distance
firstplane         r-k         t           (r-k)t
secondplane        r           t            rt
Total                                        d
</pre>

{{{(r-k)t+rt=d}}}
{{{rt-kt+rt=d}}}
{{{2rt-kt=d}}}
{{{2rt=d+kt}}}
{{{highlight(r=(d+kt)/(2t))}}}