Do you remember Trapper Keepers? With the revolutionary gutter-facing pockets and the helpful facts and all that? My brothers and sisters had those when I was wee so I learned my times tables pretty early. It always delighted me that the factors of nine were all mirror images of each other -- 18 and 81, 27 and 72, and so on. And so sometimes I just liked to sit and think about that, about how the ones and tens digits would round the bend between 45 and 54.
Sometime in the middle of elementary school, maybe, I figured out that since the difference between any two factors of nine would be evenly divisible by nine, the differences between these mirror-image factors were also always going to be divisible by nine. This delighted me even more, and for years when my mind wandered I'd just flip and divide, flip and divide. 18 and 81 were my favorites. Sometime in my early twenties I tested and discovered that it applied to all two-digit numbers, and then any number of any size, then any number of any size in any base (the difference will be evenly divisible by [the base-1]). When I'm bored I like to do these problems still; there's something very comforting about its absoluteness.
But you know what I cannot fucking do to save my life? Any math that involves years or time. I can do zero percent of that shit. Not on paper, not with fingers, not ever. I recently had to figure out when to leave to get to a 12:15 reservation 2 1/2 hours away. Relatively simple for most. Computational nightmare for me.