h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The trick is to have the numbers written down in standard form (one above the other) so that you can stare at them (back then, I didn't have to have the "x" written down so that I could remember what I was doing). Doing the multiplications in long form like we were taught in grade school is hopeless; you do cross-products and use your index finger to keep track of which group of cross-products you're doing. For example, in multiplying ABC by DEF, the first group of cross-products is C*F, the second group of cross-products is B*F and C*E, the third group of cross-products is A*F, B*E, and C*D, the third group of cross-products is A*E and B*D, and the last group of cross-products is A*D. After you finish summing up the cross-products for a particular group, you can write down a digit of the product. Remembering the carries is God-awful, but if your mind still works, it can be done.

Oops. I've got two third groups listed. So much for my memory.

Yep; all sorts of manner of different ways apart from long multiplication, which is just too hard for me to do in my head.

Anyway, nobody can say something is unprovable, except in general where everything is unprovable. Even this.

Bizarre though it may seem, it is in fact possible to prove that certain mathematical facts are unprovable.