Think of it in terms of “time” and not “speed” and it will make sense.
It is fundamentally a time, not speed, problem. Took me a min or two to wrap my head around it which is why I am not a physicist but rather an economist.
I despise Physics. Makes my head hurt. ;)
But the question is about speed, and not time. Time is not in any of the givens. Time is a factor in determining MPH, but so is DISTANCE.
The issue is how you get AVERAGE MPH, not AVERAGE TIME.
Look at it this way:
IF I go 15mph for 1 mile, and 45mph for 1 mile, what is my average speed (mph) for a 2 mile course?