Last year I figured it out from first principles, like some of the other commenters, but that problem was one of the hardest for me (like the tile rotating prob

Last year I figured it out from first principles, like some of the other commenters, but that problem was one of the hardest for me (like the tile rotating problem, which was excellent). This year, my rule is that if it looks like the Chinese remainder theorem, then I can just look it up. There's no need to stay stuck on something that's supposed to be fun.