thank you sirs by the answers, but sorry, I continue not understanding. I know this sounds easy but to me is so much complex.

The point about magical numbers if I really take the point is that it uses some form of complement, 2 complement or 1 complement I suppose, and inside computer we have 32,64, ... bits.

So, I continue not understand how that number 8 on multiplication turned into number 5 on division while number 2 remains the same on both division/multiplication by 10.

Well, let me reformulate the question:

If I have a pencil (object) and I like to divide that pencil into 3 same parts by only dividing by 2 (half). How can I do that? I reformulate this question because on pencil we cannot use 2 or 1 complement.

Is this impossible because 3 or 5 is a prime number?

Again, thank you sir by patience.