s = 1/2 at^2 = distance traveled under acceleration.
s= dist
a = acceleration
t = time

Is it constant acceleration? Can't tell from the provided information. Be nice to have interval data. Absent that, you have to assume constant acceleration. Or Make Something Up!

Anyway, acceleration = terminal velocity - initial velocity/change in time (Newtonian)
= a = (v1-v0)/(t1-t0)
a = (100-0)/(20-0) = 5 velocity units/sec

since s = 1/2 at^2 we merely substitute a and t into the equation and do the maff.

= 1/2 (5) (20^2)
= 1,000

http://en.wikipedia.org/wiki/Equations_of_motion