"one-third" VS "four-fifths"???????

其實加不加ｓ最主要是看分子，若分子大過１，分母的序數詞就必須加ｓ，用複數形。如：
One third, One quarter, One half, One fifth,two and one half(2又1/2)（因這些分子都是１，所以就不加ｓ，用單數形）
但
two-thirds, four-fifths, three and seven eighths(3又7/8)（因分子大過１，所以就要加ｓ，用複數形）

希望你明白！

The correct answer should be
one-third, two-third, one-fifth, two-fifth, etc.

one-third (noun)
one of three equal parts of a divisible whole; "it contains approximately a third of the minimum daily requirement" (WordNet® 3.0, © 2006 by Princeton University).

one-fifth (noun)
one part in five equal parts (WordNet® 3.0, © 2006 by Princeton University).

First, Second, Third, do not have an "s". Fourth and above will have an "s".

This is English Gramma. Gramma is the Law of English language. We cannot change the law but to follow the law. So, there is not an answer to your question "Why... ...". Just follow it.