Since it's an interesting problem anyway, let's see:

c = 2 * sqrt(h * (2r - h))
c = 600
h = 20

600 = 2 * sqrt(20 * (2r - 20))
300 = sqrt(20 * (2r - 20))
90000 = 20 * (2r - 20)
4500 = 2r - 20
4520 = 2r
2260 = r

So the radius is 2260. Is that what you got?