This Puzzle is not too difficult, so try to guess the answer before working it out.
A car travels at 30 miles per hour for 20 miles and the travels at 60 miles per hour for the next 20 miles.
What is its average speed?
The car has travelled half the distance at 30 miles per hour and half the distance at 60 miles per hour, so at first glance it might seem that the car's average speed is the average of 30 and 60 which is 45 miles per hour.
However speed is distance divided by time . . .
The car takes 40 minutes to cover the first 20 miles and 20 minutes to cover the second 20 miles, a total of one hour. In that hour it has travelled 40 miles, so its average speed is 40 miles per hour.