I have 5 sites with simple math captchas and get hundreds of spam a day that don't solve the captcha and which are therefore rejected. None solve the captcha.
YMMV
YMMV
Or are you simply using a simple math question (un-captcha'd)?
I've been experimenting with my own db of questions that can be answered with a word - including simple arithmetic and simple reasoning such as "Which is warmer? Hot or cold?" - and finding almost no spammer cares enough about my sites to develop a decoding table for comment submission [that would of course change if I were Amazon, etc.]
All of this is moot, of course, if some spammer were to care enough about auto-registering/emailing my site to get porn-visitors to solve the questions/captchas in order to "see more".
The formmail script then checks for the right answer.