Incorrect spelling, explanation: this is an incorrect way of writing down this ordinal number. The correct way is 20th, as the regular ending for the ordinal numbers is th. nd is only added to ordinal form of number two, so second and every number that ends with second (22nd, 42nd, 182nd, etc.). 20nd is therefore wrong and the right form to spell this number is 20th.

Correct spelling, explanation: 20th is the correct form because th is the suffix added to create ordinal numbers. There are three exceptions: 1st (first), 2nd (second), and 3rd (third). st, nd and rd are used when writing ordinal numbers only when there is first, second and third in the number (for example 23rd, 51st, 142nd). Twenty doesn’t have any of these in itself, so we end the ordinal form with regular th.

Definition of 20th:
numeral, the ordinal number between 19th and 21st
On our 20th anniversary, we are going to visit Hawaii.
The film industry was the best in the 20th century. Nowadays films aren’t so good.

Incorrect spelling

Incorrect spelling