vocabulary - How to write about ordered things?

Can anyone help me about the correct usage of nd and th ?

The nth best grade
The n'th best grade
The nnd best grade

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What are they called in the grammar ?
Which one of them is grammatically correct ?

Thanks in Advance

Answer

They are called ordinal numbers.

The first three always have the -st, -nd & -rd ending:

1^st, 2^nd & 3^rd.
In the same fashion, 21^st = twenty-first, 22^nd = twenty-second, 23^rd = twenty-third, and so on.

For upcoming numbers add -th:

4^th, 7^th, 14^th...

As for the spelling, add -th.

fourth, seventh, fourteenth.

If the number is a multiple of 10, the spelling changes:

30^th = thirtieth.