Dropping test tubes now

A test tube is knocked off a tower at the top of the Green building. (For the purposes of this experiment the tower is 400 feet above the ground, and all the air in the vicinity of the Green building was evacuated, so as to eliminate wind resistance.) The test tube drops $16t^2$ feet in $t$ seconds.

Calculate
a) the average speed in the first two seconds of the fall.
b) the average speed in the last two seconds of the fall.
c) the instantaneous speed at landing.