<p>^should be 35 instead of 53.</p>
<p>I agree, but it may be helpful to remember the harmonic mean aspect. After seeing such a rate problem many times, it is not really worth re-deriving the solution (however one must be careful that it only works when the distances are the same!). For example, a while back I had difficulty remembering the sum of a geometric series but I was able to derive it instantly, so eventually I committed to memory the formula for the sum so I don’t have to re-derive it again.</p>