Incorrect spelling, explanation: this form,
30st, is a mistake, because the suffix used to create ordinal numbers is the one that word forms of these numbers end with. Therefore thirtieth is 30th and thirty-second is 32nd. This means the only correct form here is 30th.
Correct spelling, explanation: as most ordinal numbers, thirtieth (30th) in its number form is written with -th suffix after 30. It is due to the fact, that its word form thirtieth ends with -th; according to the rule this is how we create suffixes for ordinal numbers. -st suffix is used only with number 1, as in first. Therefore 30th is the only correct form here, and
30st is mistaken.
Definition of 30th:
numeral, ordinal number between 29th and 31st
This is a 30th book I’ve read this year, and I’m very proud of myself.
I was 30th in the queue, so I have waited for 3 hours.
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