Question 715228
Your answer to part a is not quite right because your direction changed at the top of the hill, which means that your acceleration was not constant for the whole trip, up and down. Let's just solve the problem using 
(1) d = rt
Let Tup = time taken to go up the hill.
Let Tdn = time to go down the hill. 
Let d = the distance up the hill which is the same as the distance down the hill.
Using (1), we get
(2) d = 40*Tup or
(3) Tup = d/40, and
(4) d = 60*Tdn or
(5) Tdn = d/60
The average speed, Save, is given by the total distance traveled divided by the total time taken, giving us
(6) Save = (d + d)/(Tup + Tdn) or
(7) Save = 2d/(d/40 + d/60) or
(8) Save = 2/(1/40 + 1/60) or
(9) Save = 20/(1/4 + 1/6) or
(10) Save = 480/(6 + 4) or
(11) Save = 48
The average speed is 48 miles per hour.
I believe that since we instantaneously reversed directions at the top of the hill the acceleration was infinite for zero seconds, and since the displacement, d, was the same up and down, but in opposite directions the average velocity (vector) is zero. But check with your teacher on the velocity part b.