Are you human?
This is a question we often get asked when registering for things on the net - the sign up page gives a scrambled image containing some text, and we are asked to type out the characters from that.
This is to make sure that we are indeed human, and not some software created to make fake registrations for illicit usage. The idea is that software programs cannot 'see' the text within the image, while we humans can.
Now, this site has a cool new way of doing it: instead of asking to type out letters from an image, it asks us to solve a math problem. It is usually a simple problem which can be solved by anyone with undergrad level math practice. I got 'what is the least zero of the polynomial x^2 + 3x' (the answer is -3).
While not everyone might be able to solve these problems, the site registration too is not for everyone - it's a registration to download a 'quantum random bit generation service' client; someone downloading that can probably solve these problems hands down...
PS: No, I didn't download the client. I registered only because I heard about this cool method of doing a captcha and wanted to give it a try... :)