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This question already has an answer here:

Which is correct, a "two-year" program or "two-years" program?

The difference between two expressions is the absence/presence of "s", i.e., singular or plural.

marked as duplicate by snailcar Mar 29 '17 at 1:33

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When we use counted elements as adjectives, they take a hyphen and lose the plural ending -s, because adjectives don't have plural forms in English:

Here are some examples:

a two-year program, a 3-day hike, a two-hour test, a four-year-old child, a 100-year war

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    Yes, but we do actually have The hundreds year war and this is because two hundred years ago, English speakers did use plural forms of this kind of expression, as in a five-feet long piece of lumber. You'll find these in 19th century novels, for example. Basically, it is whatever usage is popular at some given point in time, as usages are always changing. – Alan Carmack May 3 '16 at 7:19
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    @AlanCarmack Agree with your comment, except that you misplaced the s in Hundred Years War. books.google.com/ngrams/… – Adam Mar 28 '17 at 17:03
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In compound adjectives that contain a noun no plural-s is used, eg a ten-year-old boy. http://www.espressoenglish.net/compound-adjectives-in-english/

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