An astronaut drops a rock off the edge of a cliff on the moon.
the distance, d(t), in meters, the rock travels after t seconds can be
modeled by the function d(t) = 0.8t^2. what is the average speed,
in meters per second, of the rock between 5 and 10 seconds after
it was dropped?
(1) 12 (3) 60
(2) 20 (4) 80