Sorry for the delay. Work was getting in the way of my goof off time.
pdurrant did get the question correct, and did a really good job of explaining how to correctly determine the correct average miles per hour. Although, Red_pheonix also got the question correct (and before pdurrant).
However, neither one of them explained why you can't just add 40 mph + 60 mph and divide by 2 to get the average miles per hour.
The reason is that Total Distance/Total Time is the
time-weighted Average Speed, whereas (speed1 + speed2)/2 is the average speed where both speeds are given equal weight.