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Twenty-one is a number. It comes between twenty and twenty-two, and is an odd number. It is divisible by 1, 3, 7, and 21.
Twenty-one is also a name of a popular card game and is the age at which people in the United States can buy alcoholic drinks.
In Roman numerals, 21 is written as XXI.
In Greek numerals 21 is written as κα