To solve this problem, it would be helpful to know the density of 1 block
Density is defined as the mass of the substance per volume.
From the example given,
The density of the block is (7g)/(15.625 units^3) or 0.448 g/units^3.
So, if a block is added, the new mass is 7g + 7g = 14 g
And the volume 14 g /(density) = 1 unit^3