Correct spelling, explanation: this ordinal number is written in its short form with abbreviation st because its full form is twenty-first. There ordinal suffixes come from the last letters of full-word written numbers, therefore fifty-fifth is 55th, eighty-second is 82nd and twenty-first is 21st. Although -th is the most popular ending (it comes with most numbers), 21th is an incorrect form.
Definition of 21st:
numeral, ordinal number between 20th and 22nd
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Incorrect spelling, explanation: this form is wrong since twenty-first as a word doesn’t end with th but with st, and therefore we should use st to end number form. This points to 21st as the correct form. th is added to numbers ending with this sound, thus 21th is incorrect.
Incorrect spelling
Incorrect spelling
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