Correct spelling, explanation: this is a correct form of this ordinal number, because word thirty do not include three in it. If number ends with three, then its ordinal form suffix is -rd, just like in ordinal number third. Thirty, therefore gets suffix -th which is the most common suffix for ordinal numbers. Thus, 30th is the correct form.
Definition of 30th:
numeral, ordinal number between 29th and 31st
For my 30th birthday I’m planning to go for a trip around Europe.
I think coming 30th in the marathon is a huge achievement.
Incorrect spelling, explanation:
30rd is a mistake, since suffix -rd is used only to number three or numbers ending with three (for example 33rd, 563rd, 1093rd). Suffix -th is the regular one for ordinal numbers and it is also used for number thirty, therefore 30th is the correct form.
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