First, find the bulb's resistance: $R_{bulb} = V^2/P = (100)^2/500 = 20 \Omega$. The current for 500W is I = P/V = 500/100 = 5 A. In the 200V circuit, total resistance needed is $R_{total} = V/I = 200/5 = 40 \Omega$. The series resistance needed is $R = R_{total} - R_{bulb} = 40 - 20 = 20 \Omega$.