Question

Calculate the radius of a segment of length 20cm which sustends at an angle 120° leave your answer to the nearest whole number ( Take π=22\7

Answer 1

If you draw a diagram, you can see that
10/r = sin60°
so r = 10 * 2/√3 = 20/√3

no need for π

Answer 2

unless, of course, if by "segment" you mean "arc" instead of "line segment"
In that case, since s = rθ
r = s/θ = 20/(2π/3) = 30/π