13's also a hard number to start with because it's a "middle" number. We tend to think and memorize numbers in sets of 10, and subsets of 5. 13, 12, 18, and 17 are in the middle of the subsets, and so are the hardest numbers for us to just grok mathmatically. It's just easier to think of numbers that are just one offset from a 5 or 10 and do the math quickly by using offsets. The farther away from a 5 or 10 number, the more thinking we have to do (and the easier it is to get confused and have to slow down and break the numbers down).
Um. We also tend to have an easier time counting "up", then down. Mentally, we can manage a number that is "one less" (19, or 14), pretty easily. We can manage "one or two more" ok as well (11, 12, 16, 17). But two less is difficult, and 3 more is difficult as well, making 13 and 18 "hard" numbers to deal with. Add in the oddness factor, and that leaves 13 as the first "worst" number we encounter (numbers in the first 10 are ok, since we tend to memorize them early on, so "3" doesn't end up being a problem, but 13 is).
The best cure for bad math skills is to play some of the more mathmatically challenging RPGs out there. It's a bit harder to find them nowadays since everthing's going to the easy d20 type systems. I used to play Champions pretty heavily back in HS. That game will cure *any* math problems you have. Today, I can still tell you whether a number is an even multiple of 5, 10, or 15 instantly, and often how many multiples just as fast (and the fractions left over if need be). Heh. And I can add up numbers on dice at ridiculous speeds at a glance.
____________________________
King Nobby wrote:
More words please