There's a cube that is made by a matter with the density $4 g/cm^3$. The edge lenght of this cube is $10$ and its mass is $3$ kg. Find the space of this cube.
So, I thought that
$$10^3 = 1000, V_{cube} = 1000$$
and tried to make them equal
$$4 = \frac {3}{1000}$$
However, It doesn't seem correct.
First of all, density is given by a unit of $g/cm^3$ but you wrote it as $kg/cm^3$. Notice that when you have a matter, its mass and volume may change but density is always constant (in this case at least). So we have $3\ kg = 3000\ g$ mass and $4\ g/cm^3$ density therefore we must have volume $V'=\frac{3000}{4} = 750\ cm^3$ by using $d = \frac{m}{V}$ where $d$ is density, $m$ is mass and $V$ is volume. Therefore we have $1000-750 = 250\ cm^3$ space inside the cube.