Question

how to find the radius of a ballon?

Answer 1

I'd need more details to be able to give you a sensible answer. What do we know about the balloon in question?

Assuming that the balloon is more or less spherical, and that you can measure its diameter, then the radius is half the diameter.

Since the volume of a sphere is V = (4/3)*Pi*r^3, r^3 = 3V/(4Pi), so that the radius, r, is the cube root of 3V/(4Pi).

So, any response you get to your posting will depend upon the reader's own conception or interpretation of what is to be done.