At first, I couldn't do it in my head, either, and its a very simple problem, solvable by trial and error. I could remember the algebraic procedure to find a solution (although it was not necessary for such a simple problem) and it certainly wasn't a method you'd expect a third grader would know.

I've been catching myself having these lapses lately, like getting lost in long division, or giving up on a simple paper and pencil calculation and using the calculator instead. Is it age, or is it something worse? I can usually recall procedures fairly easily (like solving simultaneous equations) or techniques (like collecting terms, or organizing by powers of ten and isolating exponents) but find myself getting stuck on really simple things like forgetting parts of the multiplication tables.

I know I haven't used any of this stuff systematically in a long time, but I never imagined I would lose this much, so quickly. It's scary, this used to be such a huge part of my life, and now I feel it slipping away.