The silver bar is 15cm long. The bar is melted and the silver is then made into a cuboid. The base of the cuboid is 10cm by 10 cm. What is the height of the cuboid?

The question is impossible to answer since. Since you can't answer that without knowing the volume of the bar to start with. The only given is the length, there should be other dimensions like the width and area of the cross-section, for instance.

Volume = Width x Depth x Height in all cases, so when you know the volume. But to solve for the height of the cuboid, the equation is V/100 (because the base of the cuboid is 100 cm^2)