Math Captcha?

I am not sure what’s going on. My confusion comes from two things that as far as I know are correct. The 1st one captchas exist so we can tell if the user is human or machine. The second is that computers are good at doing calculations. I am right? Right?

So, if I am right… why is this a good idea?

Captcha

I am right? Right? What am I missing here… I could eval that with js easily to get an answer… and I’ve seen this captcha in more than one place.

I don’t know maybe there is some hidden technique I am missing. I would like to hear from you if you can explain.

Edit: The other version of this is asking in words something like “What is five plus two?”… have you ever typed that into google…? Google is a machine and it knows. Seriously, what’s going on here?