The displacement (in feet) of a certain particle moving in a straight line is given by $s=t^3/6$, where $t$ is measured in seconds.

(a) Find the average velocity over the given time periods:
(1) [1,3]
(2) [1,2]
(3) [1,1.5]
(4) [1,1.1]

(b) Find the instantaneous velocity at $t=1$.

(1) ft/s
(2) ft/s
(3) ft/s
(4) ft/s
(b) ft/s