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We could just randomize it so it's 17 + 21, and then just tweak the way it is said so it becomes thing likes "one before eighteen plus one past twenty"

Wow, I would not pass such captcha on a first try :). It sounds like the french way to call numbers, which I believe, is not very common.

Oh, hrm, fair point. French numbers are way more difficult then this though. This is just offset them by 1 each.