Adding Simple Math Based Security Question to Contact Form

I have six landing pages (single page) that have a contact form. the landing pages are already done. I only need the security question part implemented. To combat spam, I would like to implement a simple captcha. (See image). It's a simple math problem that needs to be "Solved". the math problem should rotate between several "math problems" to make it less likely that a bot will be able to quickly solve. Here is a suggestion on how to implement this: Here's what I've found to be very effective (and dead simple): Put a hidden field on your form. Give it a name like "phone" or something similar/common and put in a default junk value. Put another regular text input field on your form, but hide it with CSS. Make that one empty. Again, give it a "real" sounding name (first_name, phone_number, whatever). When the form is posted, verify that the hidden field still has the default value and the field you hid with CSS is still empty. You're basicly taking advantage of the fact that most spam bots will simply fill in every field in the form in order to avoid failing any required field validation checks. Some might be smart enough to ignore hidden fields, but I've never seen one that was smart enough to ignore fields hidden with CSS. I'm open to suggestion how to implement this. Put in a bid for the project and I'll post the exact landing pages that need to be worked on.