Since diameter of the sphere = 20cm, then, the radius of curvature = 10cm

Focal length, f = \(\frac{R}{2} = \frac{10}{2}\) = 5cm

When the inner/ inside portion of a sphere is polished/coated/painted, the outside becomes the reflecting part and the resulting mirror is called a convex mirror.

Thus, a convex mirror of focal length 5cm is formed.