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?
It's impossible  you'd have to travel at infinite speed.

Posted by e.g.
on 20040310 08:41:47 