I'm assuming that the list itself is the right place to deliver solutions!
Here is mine, for what it's worth.
http://www.holygoat.co.uk/lisp/cl-quiz/captcha.lisp
* (generate-captcha) "what is 7 minus 3?" "4"
* (generate-captcha :elements 4 :depth 1) "what is 8 times 7 times 1 times 6?" "336"
It's a bit kludgy -- disrespecting as it does infix operator precedence, so (> depth 1) is unlikely to calculate correctly -- but it was fun to write! A good Sunday morning's finger exercise.
-R