Correct spelling, explanation: 30th is the correct way to write down this ordinal number because the full-word form is thirtieth. Therefore letters th which are at the end of the word, become the suffix for a number: 30th. The uffix -st is then used with the ordinal number first, because the word ends with st. This is why 30st is the wrong form.

Definition of 30th:
numeral, the ordinal number between 29th and 31st
Her 30th birthday party will take place in the lake house.
-When is their wedding? -On July 30th.

Incorrect spelling, explanation: 30st is a mistaken way to write this number. This is because -st is a suffix that we use with the ordinal number first (because first ends with st). Thirtieth (30th) ends with th, so such a suffix should be used to write down this ordinal number. Therefore the correct form is 30th.

Incorrect spelling

Incorrect spelling