"Forty foot" or "forty feet"? [duplicate]
Solution 1:
The forty-foot drop is correct.
Compare this:
1) He's a 10-year old.
2) He's 10 years old.
In the first example, a 10-year old stands as a noun (notice the article preceding it). In the second example, it's not a noun, therefore no article.
Therefore:
1) It's a 40-foot drop.
2) This drop is 40 feet in height.