Well let me help you here. My suggestion is that you use your favorite technique to find a smooth curve that fits the data points
reasonably well. Lets remember that the t values are equally spaced, I'd use Newton's forward difference formula
It tells you that s(t) = 12.5 + 30t - 6t², and this fits the data perfectly.
So s'(t) = 30 - 12t,
Apply what im telling you and yu will get to the result