It’s true that a bot can be specialised to solve it, but i feel that is the case no matter what you do.

To me the appeal of this approach is that it is very simple for a human to make the rules (e.g. numbers with two digits are harder to add than numbers with one digit, or "the more leading letters two words have in common, the harder they are to sort) but for a bot to figure out the rules by trial and error (while answering at human-like speed) will take time. So the set of questions can be changed quite often at low cost, making it less feasible to re-train the bot every time.

Another alternative could be to only give questions that are trivial for a bot, but annoyingly difficult for a human, and let them through if they press “reset captcha” a couple times, though some people might find that annoying…