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All answers having "Twenty-one" as clue:

4 Character Answers:

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GAME



5 Character Answers:

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OFAGE



9 Character Answers:

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BLACKJACK



10 Character Answers:

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TVGAMESHOW



15 Character Answers:

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FIVEPLUSSIXTEEN

SEVENTIMESTHREE

SIXTEENPLUSFIVE

THIRTYMINUSNINE

TWOINTOFORTYTWO