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:

1. random spambots and the human spambots that might back them up; and
2. bots created to game StackOverflow

With simple arithmetic, you might beat off threat #1, but not threat #2.

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:

1. random spambots and the human spambots that might back them up; and
2. bots created to game Stack Overflow

With simple arithmetics, you might beat off threat #1, but not threat #2.