xkcd #2937: Room Code ( imgs.xkcd.com )

lowleveldata , 1 month ago

xxx518

The second half is 3 digits. I'd just hard code memorize it.

0x0518

The first half is something between 0. It's a easy pattern.

020518

The last missing digit must be something simple and deductible from other numbers if I put it to the last. As all other numbers (0, 518) are even, it should be 2.