Here's a baby-talk approach that should make it clear . . .
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.