Math:

"Write down numbers by using ten symbols called digits for the numbers zero through nine, and writing XY, where X is a number and Y a digit, for the number equal to ten times the number represented by X, plus the number represented by Y."

Maybe this could be optimized by picking a better base to work in (2? 8? 12?), but if post-apocalyptic humans have the same number of fingers, then base 10 has its advantages as well.