Beware not to mix up fractions (to describe a part of a whole: half, third, fourth...) and ordinal numbers (to describe an order/rank: 1st, 2nd, 3rd, 4th, ...).
For ordinal numbers, see Forming ordinal numbers (1st, 2nd, 10th...).
Look at these examples of fractions:
un cinquième1/5 - one fifth
deux cinquièmes2/5 - two fifths
seize vingtièmes16/20 - sixteen twentieths
To express fractions in French, you usually use both cardinal (un/deux/trois...) and ordinal numbers (premier/cinquième...) the same way as in English (one fifth).
Note also that just like in English, the second number is plural if the first is.
Here are the irregular fractional numbers
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