I think it's like this:

Let a = initial time, b = final time

average velocity is V = (1/(b-a))*Integral(v(t)dt)

where the limits of the integral are a to b.

When you're given s(t), your job is made easier because the above equation turns into

V = (1/(b-a))*Integral((ds/dt)dt) = (1/(b-a))(s(b)-s(a))

I hope everything is clear.. I've been finding LaTeX a little cumbersome lately so I sometimes use it, sometimes not.

Someone please correct me if this is wrong as I have not worked with these for a while.