Thanks for the plugin ! It's working fine on my custom form, sadly sometimes the result displayed is null. This seems to happen with a captcha of this format:
number operator box = null.
Example : 1 - box = null
It's either an issue in the math or with the language of my wordpress (french). Maybe the result should be written in letters but can't find a match in my language so it returns null. I will try with an english version of wordpress.