Paul & John leave city A for City B at the same time.

After reaching City B, both return to city A immediately.

Paul travels from A to B @ 56 kph and returns @ 64 kph.

John travels @ 60 kph both ways.

Who gets back to A first?

. (i) Paul

. . (ii) John

. . (iii) Both arrive at the same time

. . (iv) Cannot be determined

Now my prof says JOHN arrives first as the average speed of Paul (the harmonic mean)

will be less than the arithmetic mean (60kmph) ...

Brilliant . . . but it'll take him an extra hour to explain all that.