When you have a number with a noun used as a modifier for another noun, the adjective-noun modifier should not have an “-s” ending. Consider the phrase “thirty minutes.” In this case, “thirty” modifies “minutes.” The word “minutes” should have an “-s” ending to express plurality. If we take the number + noun and make it into a modifier, the noun will no longer have an “-s” ending. For example, “a thirty-minute drive”. Now, the word “thirty” is NOT modifying “minutes”; “thirty-minute” is modifying “drive.” In the following examples, make note of when to use and when not to use the “-s” ending.

• It was a three-mile hike. vs. The hike was three miles long. 
• She is an 11-year-old girl. vs. She is 11 years old. 
• This is a four-passenger car vs. The car can hold four passengers. 

• I bought a five-foot pole. vs. The pole I bought was five feet long. 
• This is a two-person ride. vs. This ride can hold two people at a time.