X/5120=20/100
x/5120=1/5
5120/5=1024
1024+5120=6144
6144 is the answer.
I wrote it out on paper and uploaded the answer for you. Let me know if you have any questions.
The Shaded area is .25 of the circle, so:
Area/4 = Shaded
18/4 = Shaded
4.5 = Shaded
Hope this helps!