On number two of the second file, mass is how many atoms there are, while weight is how much he gravitational body the object is on (if any) pulls the object (which is why we weigh nothing in space).
For the first question of the second image, A's answer would be 1,800 g/300cm^3, or 600g/cm^3.
For B, .250g/cm^3 * 200cm^3 is 50g.
For the table: Density is how compact the matter of an object is in a certain amount of space. An example would be that water has a density of 1g/cm^3
Mass is how much matter there IS, for example, a car has more mass than an orange.
Volume is the quantity of 3d space an object has or how "big" it is. For example, a cubed building is the length of one side to the third power
Weight is how much gravity is pulling on an object, and not related to how much stuff an object has other than that, for example, you might weigh 100 kilograms on Earth but none is space, and your matter hasn't changed.
I hope you enjoyed your answer.