SOLUTION: A driver sets out on a 20-mile trip. When he has gone halfway he finds he has averaged 25 mph. At what speed must he travel the rest of the way to make his overall average speed fo

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