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 Over complicated by alex)

Thinking of it as a simple grade 6 math averaging equation is just wrong.

To go 40 miles (the round trip) at an average speed of 30 mph takes 1 hour 20 minutes. Taking longer than that would mean travelling at less than an average speed of 30 mph.

But the 20 miles at 15 mph has already taken 1 hour 20 minutes.  The time is all used up.

The 6th grade teacher never taught about the correct use of averaging or how to do harmonic means, (not to mention geometric means, useful in other problems.)

Posted by Charlie on 2004-10-10 18:59:06