P(t) = P0 + Integral from 0 to t of P'(t). You can think of the integral from 0 to 20 of P'(t) as the change in the population over 20 years from the original population in 1990 (i.e. the change in population from 1990 to 2010).
1995 to 2000. Integrate from t = 5 to 10
2020 is t = 30
P(30 = 10000 + integral from 0 to 30 of P'(t)
Indefinite integral of 200e-.02t is( 200/(-.02))e-.02t = -10000e-.02t + c
Hope that helps.