As a general rule, use the singular form of the words when they are used as ADJECTIVES (Pang-uri in Filipino); when the words are used as NOUNS (Panggalan in Filipino) use their plural form.
1. Kris Aquino signed a three - year contract with ABS-CBN. (adjective)
2. His employment contract abroad is for two years. (noun)
3. Ana has a twenty-dollar bill. (adjective)
4. Watermelon costs three dollars a piece. (noun)