Use the Compound Amount formula:
A = P(1 + r/n)^(nt), where P is the principal ($200,000), r is the interest rate as a decimal fraction (0.045), n is the number of compounding periods (240), and t is the number of years (20).
In this case
A = $200,000 ( 1 + 0.045/12)^(12*20)
= $491,093.27