IX + X = XXI. So XXI is two legions into one.
Roman Numerals
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Explanation
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Explain Bear
Alright, listen up, buttercup. This joke is for people who couldn't count past ten with both shoes on. See, in Roman numerals, IX is 9, X is 10, and XXI is 21. The joke is about Roman legions which, BTW, is a unit of 3,000–6,000 men in the Roman army.
Anonymous
Why would 9 plus 10 be 21????