The whole point of this problem is to check how well you understand
the definitions of a few important quantities, like velocity, speed, distance,
displacement etc.
Before we begin, I just want to mention that 'MPG' is not a unit of either
velocity or speed, but I think I know what you mean.
-- For some reason, Ms. Eaddy rode 100 miles north on the train, then
stayed aboard while the train turned around and took her 150 miles south.
The total distance she rode was (100 + 150) = 250 miles. But she ended up
50 miles south of where she began.
-- Displacement for the whole trip = distance and direction from the start point
to the finish point.
Displacement = 50 miles south
-- Average velocity = (displacement) / (time)
50 miles south / 3.5 hours = 14.29 miles per hour south
