Learn English – word for opposite numbers

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• Americans pronounce fractions with denominators ending with 1, 2, 3, as in twenty-firsts, twenty-seconds, twenty-thirds. For confirmation, here is a definition from Merriam-Webster, one of the canonical American dictionaries.

thirty-second 2 : the quotient of a unit divided by 32 : one of 32 equal parts of anything <one thirty-second of the total>

The word thirty-twoth does not appear in the Merriam-Webster dictionary, no matter how you spell it. While a few people may use thirty-twoth, it's definitely non-standard.

• There is a difference when the denominator is 4. Americans use both fourths and quarters for one piece of something divided into four equal parts (except for hours, which are always quarter hours) while in the U.K., these are usually quarters. Oxford Dictionaries Online has

fourth 2. chiefly North American A quarter: 'nearly three fourths of that money is now gone'

Merriam-Webster has both

fourth : one of four equal parts of something
quarter : one of four equal parts of something

I think a better way to describe the relationship you're looking for is: Term for two synonym phrases that use antonyms; ie: I'm up for that / I'm down for that. In this example, "up" and "down" are antonyms, but they both contextually mean "willing/able."

I was unable to find a discrete term for the phenomenon, considering I was unable to come up with any examples that aren't slang, I doubt a simple term for the specific case exists. Off the top of my head: "paradoxical synonym phrases?"

Edit: You're not alone Opposonym (Suggested on wikipedia but not anything formal) and Paradoxical Heteronym