Mistaken insertion or omission of hyphens in phrasal adjectives that involve quantities is a common error. The following sentences illustrate several types of incorrect usage to avoid.
1. In his most successful season, he made 13-of-16 field goals.
In the context of sports, in a simple reference to a number of attempts achieved, hyphenation is extraneous: “In his most successful season, he made 13 of 16 field goals.” The exception is when an “x-for-y” phrase stands on its own (as an adverb) in place of an “x of y” phrase serving as an adjective for a noun or noun phrase describing the results: “Smith went 4-for-5 to lead the team to victory.”
2. The team’s efforts to repeat the successes of the previous season were stymied by a 57-day long players’ strike.
This sentence describes a long players’ strike consisting of 57 days, but that’s not quite what it is intended to mean. The strike was 57 days long, and that combination of numbers and words, not just the first two elements, constitutes the phrasal adjective describing the players’ strike: “The team’s efforts to repeat the successes of the previous season were stymied by a 57-day-long players’ strike.”
3. The 6 acre town is situated in a small valley between rolling hills.
The number and the unit of measurement together provide information about the town, so the two elements of this phrasal adjective should be hyphenated: “The 6-acre town is situated in a small valley between rolling hills.”
4. Smith is still expected to rake in enough votes to clear the 15-percent threshold to get a share of the delegates.
An expression of a percentage that modifies a noun, unlike a similar-looking reference to a dimension (“15-foot threshold”) or a dollar amount (“fifteen-dollar threshold”), is not hyphenated: “Smith is still expected to rake in enough votes to clear the 15 percent threshold to get a share of the delegates.”
5. A study says that his grammar skills are equivalent to that of 10 and 11 year olds.
This type of description, in which only the head (the first element) of a phrasal adjective appears when a parallel and complete phrasal adjective follows (indicating that the two heads share the body that follows the second head), is often styled incorrectly. Because the correct format is “x-year-olds” (with an implied noun following this phrasal adjective, making the phrase itself a noun), the full phrase should be hyphenated, and a hyphen should follow the first head to indicate that it shares year and olds with the second number: “A study says that his grammar skills are equivalent to that of 10- and 11-year-olds.” (Note that letter spaces precede and follow and.)