For parts (a) and (b), each is a different application of Mean Value. Pay attention to the given information and read carefully.
See Math Notes below.

Average Velocity = 16.102 mph

Average Velocity = 19.907 mph

This is an application of Mean Value Theorem, which states that a function (such as a velocity function) must equal its average value at least once on a closed interval (that is, if the function is continuous and differentiable).
See Math Notes below.