A bit short in my opinion, but quite satisfying.
@MercutioSargtlin (and maybe others): if you have not find a logic for one of more of these riddles, it is entirely because of yourself. Do not blame the game, it is well-thought and the puzzles are great.
MAJOR SPOILERS AHEAD
PLEASE READ ONLY AFTER FINISHING THE GAME
For all the people who do not get puzzles 4, 5 and 6, there is still a rigorous logic in them. In number 4, you have to understand what the entirely fictional operator # does, which is a pretty common kind of puzzle, and what it does is summing the numbers and adding 2. In number 5, there is no "screwing with the order of mathematics", as I read in some looooong comment below. There is a quite simple pattern: in every group of 4 numbers, it is strictly the same logic, that is: the product of the numbers on the left and right, and the product of the numbers on the top and bottom, are the same. Finally, number 6 makes sense: a one-digit number, another one-digit number, and their product with the first digit hidden).