03 Jan 17
The square root of 600m = (approximately) 24.4948974278318.
Big goat circle (BC) minus little circle (LC) must equal half the little circle. The area of the little circle is 400pi so the area to be eaten by the goat must be 200pi.
So we have Big Circle - Little Circle = 200pi. Little Circle = 400pi so our equation is BC - 400pi = 200pi. BC = 600pi.
But BC = pi * radius squared so our equation is pi * r squared = 600pi. We simplify by eliminating pi leaving the Big Circle radius squared = 600. BC radius = square root of 600 = approximately 24.4948974278318.
Good?