[realaravinth]

The math is performed by the browser. All the user will have to do is tick a checkbox.

Here's a demo: https://demo.mcaptcha.org/widget/?sitekey=pHy0AktWyOKuxZDzFf...

disclosure: I'm the author of mCaptcha

[_448]

Commercial spammers use high-end resources, where as users don't have that luxury. So will it not be easy for the spammers to scale the resources to overcome this?

[web007]

It's not that they can't scale, it's that you want them to HAVE to scale, to make it more expensive to attack your site than another one.