ICOT,
Total travel time must be a factor. Let’s say the total trip time is 1 hour and 7.5 minutes (1 and 1/8 hours), then the same trip took an extra 7.5 minutes – is it possible that the speed was the same? Obviously not. The amount of time spent driving in one direction hasn’t changed (it was still 60 mph), but the amount of time spent driving back is 7.5 minutes longer. The distance was the same so therefore, the speed must have been lower.
You must have originally come to the same 36 MPH conclusion as I by using a 1 hour travel time when running your numbers. In the latter example (a total trip time of 1 hour and 7.5 minutes), the speed on the return trip will be 30 MPH.
As far as the formula is concerned, I’d be remiss if I failed to point out that a similar riddle was submitted a couple of years ago ==> over here <==, and I used the same formula as was to solve that example.