In case anyone's wondering why these days if you are pleasant enough to leave a comment on this site you are challenged with a mighty maths question (such as 2+2=?), then the answer is that it is an anti-spam measure. It is effectively a CAPTCHA test, but unlike the classic "what letters do you see in this picture?" it is usable by people surfing without images.

The downside is that it would also be easier for the spam-bots to pass this test that the more visual ones, but spam so far has gone down from 70-100 comments a day to zero.