You have two identical boxes which an interior dimensions of 8.0x8.0x8.0. You completely fill one of the boxes with wooden spheres that are 1.6 in diameter. The other box gets filled with wooden cubes that are 1.6 cm on each edge. After putting the lid on the filled boxes, you then measure the density of each. Which one is more dense?

First, the box filled with spheres will have empty space, this is for how spheres are, so independent of the radius of the sphere, we will always have some empty space inside the box.

In the case of the cubes, this does not happen. Furthermore, the dimension of the box are 8cm by 8cm by 8cm, and the cubes have a sidelength of 1.6cm

We know that:

1.6cm*5 = 8cm

Then we can perfectly fill the box with the wooden cubes (such that there is no empty space like in the previous case).

Because this box has no empty space and is filled with the same material as the other box, we can conclude that this box will have a larger mass than the one filled with spheres.

And we know that:

density = mass/volume.

Both boxes have the same volume, but the box with cubes has a larger mass, then the box with cubes will have a larger density.