In order to average 40mph on a 20 mile trip, the time his
total trip must take is found by:



So in order to average 40mph he must arrive half an hour after he left.

Now we'll calculate how much of that half hour he has already used up 
during the first half of his trip, or 10 miles:



Therefore he has only  or  or  of
an hour left to travel the remaining 10 miles.  Now to find
out what rate he must travel in that remaining hour,
we use:



Edwin