Explanation:

Given,
Internal diameter of hollow sphere = 4 cm
Internal radius of hollow sphere = 2 cm
External diameter of hollow sphere = 8 cm
External radius of hollow sphere = 4 cm
Volume of the hollow sphere = (4/3)∏×(4³−2³)…………………..(i)
Given, diameter of the cone = 8 cm
Radius of the cone = 4 cm
Let the height of the cone be x cm
Volume of the cone = (1/3)∏×4²×h……………………………..(ii)
Since the volume of the hollow sphere and cone are equal.
So, equating equations i and ii,  we get,
(4/3)∏×(4²−2²)= (1/3)∏×4²×h
H= 12 cm
The height of the cone so formed is having a height of 12 cm