DAY 11

Comment

Altogether, the bottom two layers of 1 cm cubes contain the same number of blue cubes and white cubes.
So do the next two layers, and the next two.
This leaves the top layer ...

Answer

There is 1 more blue cube than white cube.

Readers' methods

Misha N, Peterlee SR8:
There are 7 x 7 x 7 = 343 small cubes altogether.
There are 25 columns containing 4 blue cubes and 24 columns containing 3 blue cubes,
which is 100 + 72 = 172 blue cubes altogether.
So there are 343 - 172 = 171 white cubes.
172 - 171 = 1.

xxx