S(t) = 8t^2 - 9t + 20, v(5) = ?

The velocity function v(t) is the derivative of the position function S(t).

v(t) = dS/dt = d/dt (8t^2 - 9t + 20) = 16t - 9.

Now, substitute t=5 into the velocity function: v(5) = 16(5) - 9 = 80 - 9 = 71.