Wow, I did not expect this game to be this interesting at first! I managed to open the safe with one attempt remaining, after getting both of the totally overpowered "odd/even digits to 0" cards into my deck. I really liked the different operations that were available, altough I think the "-X" cards are not that useful in comparison to the other cards, as they really only affect the rightmost digits and you generally need multiple of them to get just one digit down to 0, which can take a few turns compared to a "set to 0" card which is also very likely to be pulled for the right digits.
Very creative!