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 :
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 suffix should be used to write down this ordinal number. Therefore the correct form is 30th.

Incorrect spelling

Incorrect spelling