You need to go from point A to point B, and then back to point A. Points A and B are 20 miles apart. You go to point B at a constant speed of 15 miles per hour. If you want your overall speed to be 30 miles per hour, how fast would you have to go from point B back to point A?
(In reply to why distance??
by Ady TZIDON)
This is generally referred to as a "red herring", and it holds a
time-honored tradition in problem telling (of course, to lead
would-be-solvers down a dead end).