What is the density of an object I know the formula, but I don't know how to calculate it. How to calculate it is different from the answer. How to multiply (divide) numbers? The unit is right!

What is the density of an object I know the formula, but I don't know how to calculate it. How to calculate it is different from the answer. How to multiply (divide) numbers? The unit is right!

Do you divide the mass and volume of two different objects?
Or do you want the density of a solid, but the object is not solid?
Otherwise, the answer is wrong!
If you need to, you can send the questions up and let's do it together~

Second grade physics, volume and density calculation and application of objects
An iron ball with a volume of 40cm and a mass of 156g. If the iron ball is hollow (the density of iron is 7.8x10 and a mass of 179 kg / M), find: (1) the volume of the hollow part; (2) the mass of the whole ball if the hollow part is filled with water

Note here that it is much easier to choose g / cm ^ 3 as the unit of density: 1g / cm ^ 3 = 1000kg / m ^ 3 ρ water = 1g / cm ^ 3 ρ iron = 7.9g / cm ^ 3 below v iron = m iron / ρ iron = 158 / 7.9 = 20cm ^ 3V ball = 40cm ^ 3V air = V ball - V iron = 20cm ^ 3 conclusion hollow m water = ρ water * V air = 1 * 20 = 20GM total = m iron + m water = 158 + 20 = 178

If the volume ratio of the above water part to the underwater part is 3:7, what is the density of a cube block kg / m ^ 3

0.7