There: 50 mi at 40 mi/h = 1.25h,

Back Again: 50 mi at 60 mi/h = 0.8333... h,

There and Back: 100 mi, taking 1.25h + 0.8333... h = 2.08333... h

Speed: 100 / 2.08333 ... = 48 mph DONE

By the use of my trusty calculator.

Who thought it should be the average of 40 and 60 = 50 mph ??