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?

Going from A to B at 15 mph takes 80 minutes.

Doing the whole round trip (40 miles) at 30 mph would also require 80 minutes.

So, in order to manage the 30 mph average, you'd have to do the return trip in 0 minutes flat, which would require infinite speed.

Posted by e.g. on 2004-03-10 08:46:24