When I was in elementary school, I was terrible with math. Easily my poorest subject. Having an interest in science, I knew I had to become better with numbers, so fluency with math became a goal. It didn't happen overnight, but it did happen, and by the time I hit high school I was considered something of a math whiz. Or perhaps it was idiot savant, I can't recall.

Anyway, it wasn't a matter of overcoming a disorder, it was a matter of attaining (and retaining) the required skills.

Anyone who doesn't either work or play with numbers will get rusty with them after a while. Hence figuring a tip or proper change may become something of a challenge. It doesn't help that those people may also cramp up a bit further with math-phobia.

I'm a firm beleiver in the notion that anyone can become, and remain, fluent and relaxed with numbers if they simply exercise their numerical abilities once in a while.