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 Answer by K Sengupta)

Suppose the distance between point A to point B is 20 miles(given).

The individual goes to point B covering x miles at a constant speed of 15 miles per hour, and the  duration of the trip from A to B is 20/15 = 4/3 hours.

If the individual were to achieve an overall speed of 30 miles per hour, then the duration of the total trip from A to B and back, covering a distance of (20+ 20) = 40 miles is 40/30 = 4/3 hours..

But in that situation, the duration of the return journey from B to A would be 4/3 - 4/3 = 0 hours, which leaves the individual with practically no time to even commence the return trip from B to A.

Consequently, it would be impossible to achieve an overall speed of 30 miles per hour.  

Edited on June 17, 2008, 1:32 pm

Posted by K Sengupta on 2008-03-28 05:53:05