What is the radius of the spherical cavity?

[ramalik]

An irregularly shaped chunk of concrete has a hollow spherical cavity inside. The mass of the chunk is 38 kg, and the volume enclosed by the outside surface of the chunk is 0.025 m3. What is the radius of the spherical cavity?

I think you have to find the surface area to solve for the radius and all this has to do with the density of the sphere, but I don't know how to connect it all

 

[Doc Al]

No need to mess around with surface areas, but you do need to use the density of concrete. (Look it up!) Hint: If the chunk were solid concrete, what would be its mass?