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Wolfram Alpha does solve some of them. It seems easy to detect the type of question and manually implement a way to parse each.

Here's an example that I got while trying to register an API account: https://www.wolframalpha.com/input/?i=One+%2B+3+equals+%3F

Same. Very easy to identify human vs. non human.

Do give an example.

http://textcaptcha.com/demo

Randomly returning a word from the question will get my bot in about 3% of the time which is plenty.

Seems like it would be fairly easy to parse many of these questions automatically. The only reason they work is probably that they're not widely used enough for spammers to care about.