Very simple arithmetic is good. Blind people will be able to answer. (But as Jarod said, beware of operator precedence.) I gather someone could write a parser, but it makes the spamming more costly.
Sufficiently simple, and it will be not difficult to code around it. I see two threats here:
- random spambots and the human spambots that might back them up; and
- bots created to game Stack Overflow
With simple arithmetics, you might beat off threat #1, but not threat #2.
