A holllowspherical ball whose internal diameter and external diameters are 8cm and 10 cm is just float in a liquid of specific gravity 1.5 - What is the specific gravity of the liquid?

You've stated that the SG of the liquid is 1.5. Do you need to find the SG of the material the sphere is made from? The way to work this out is to find the amount of the material used. The volume of a sphere is 4/3*pi*r^3. So find the volume of the entire sphere and subtract the volume of the hollow bit. Now you know the volume of the liquid displaced, (it's the entire sphere) and using Archimedes you know that the "upthrust experienced by a body in a fluid is equal to the weight of the fluid displaced". So you know the relative weight of the material in the sphere ('cos it JUST floats). Voila! You can calculate the SG of the material the sphere is made of!