He cannot possibly average 60 miles per hour, even if he could return
on a jet plane traveling 500 miles per hour. 

Here is why:

Suppose it is 30 miles from A to B.  Then to average 60 miles per hour
for the round trip, he must go 30 miles and return 30 miles in 1 hour.
He has just one hour to go and return.

But since he goes at 30 mph, he has already used up the entire hour just 
to get to B.  So there is no way he can average 60 miles per hour.

Edwin