What do you call this fraction?

Is this how you write these fractions in words?

5/21 is five twenty-ones, 1/21 is one twenty-one. Can someone please clear this for me?

I know 1/4 is one-quarter or one-fourth and 2/5 is two-fifths and 5/25 is five twenty-fifths.

Solution 1:

You will find that dictionaries like this one use the ordinal to define the fraction:

being one of 21 equal parts into which something is divisible <a twenty-first share of the money>

The courts in New Jersey have no trouble using the twenty-first part of estates divided per stirpes:

Therefore, upon the death of Frank, one-seventh equals three-twenty-firsts passed to his son. Anna, dying childless, her two-sevenths equals six-twenty-firsts passed to Mary , Henry, and William equally; two-twenty-firsts to each.