Car Journey

Originally posted by Mephisto2
In real life, one spends less time going back and forth on a flat road than on a road with hills. Which would be obtained more likely with with the average of the two speeds.

Where I live it is the opposite.

Anyway, the bigger the difference between the uphill speed and the downhill speed, the less accurate will be your calculations.

Originally posted by THUDandBLUNDER
Where I live it is the opposite.

.

far from earth?

Originally posted by Mephisto2
In real life, one spends less time going back and forth on a flat road than on a road with hills.

I thought you meant you live in a hilly area.
The guy in the puzzle may live in Kansas. 😉

Originally posted by THUDandBLUNDER
That is correct, davegage!

He can work out the distance from A to B if he takes the same amount of time to go x miles flat & back as he does to go x miles uphill and x miles downhill.

To put it another way,
if
the downhill ...[text shortened]... /(d+u)

That is, d, f, u, must be in harmonic progression.

Or...

Average speed = Total distance / Total time

Let d be distance between towns A and B.
Let t be time.

Then, t = d/72 + d/56 = 2d/63 (after cancelling)

So, Average speed for whole journey = 2d/(2d/63)

= 63mph