Have you started by drawing a diagram of a chocolate bar?

Hints:

- The volume of a prism is equal to the product of its cross sectional area with its height.

- Surely if you know the volume and you know the mass per cubic centimetre, you can find the mass of the chocolate bar.

- To find the surface area, a good strategy is to draw a net of your shape and evaluating the area of the net.

- How could the chocolate bars fit together? What would be the dimensions of this resulting shape?