Using SOH-CAH-TOA, we have 5 triangles, each with a hypotenuse of 20, which split the pentagon using lines connecting the center to the pentagon's sides.
If we split each of these triangles in half with a perpendicular from the center to each side, we have 10 right triangles. The area of each is (1/2)*20*sin(36 degrees)*20*cos(36 degrees) = 951.06 square mm, rounded to 2 decimal places of accuracy.