Most of the Marketing Bots are advance enough to break through any type of Captcha Code. The real issue is not pondering over your captcha code, but you need to make your login/registration process secure with additional questions such as
What is the 3rd character in my Domain after www. ?
As far as math captcha is concerned, it can be easily cracked by Marketing Bots. You need to create your own custom question and this will surely help to get rid of automated registrations.
I was handling wordpress blog for 1 client and every day I used to get around 10-50 comments. Ultimately I had to block the commenting feature. Spamming is really bad form of marketing.