Two positions of a block are shown below:



When six is at the bottom, what number will be at the top?

In both positions, the top face is common and contains 5 dots.
The faces adjacent to the face with 5 dots are 2, 3, 4, and 6.
Since the common face (5 dots) is in the identical position (top) in both diagrams, the remaining corresponding faces are opposite to each other:

* The front-left face of position (i) corresponds to the front-left face of position (ii) $⟹$ 3 is opposite to 4.
* The front-right face of position (i) corresponds to the front-right face of position (ii) $⟹$ 2 is opposite to 6.

Thus, the face opposite to 6 has 2 dots. When 6 is at the bottom, 2 will be at the top.