It so happened that we had a very good math teacher, who was a Communist. I remember having learned from him something that I never forget: the rule of nines. A simple point: you add up the numerals in a number, and if the original number was divisible by nine, then the sum of the figures also is. For instance, you take a number like 243. Two and four and three is nine. Therefore, 243 must be divisible by nine. Actually it is nine times 27. The rule is interesting because it’s so simple. What was really interesting to us ten-year-olds is that our math teacher proved it. The proof is not terribly difficult, but it was one of the first simple and not quite obvious mathematical proofs that I encountered.
Recorded Academy of Achievement interview source ↗