Writers are easily confused by, or are negligent about, proper use of hyphenation with phrases with numbers, whether the numbers are represented in spelled-out or numeral form. The following sentences represent various types of erroneous use of hyphenation; a discussion after each one points out the problem, and a revision resolves it.
1. In April 2016, the Houston area was soaked by a once-in-10,000 years rainfall event.
This sentence, which refers to a rainfall event of the type that occurs once in 10,000 years, includes a phrasal adjective representing that frequency, and year is part of the phrase, so it must be connected to the rest of it: “In April 2016, the Houston area was soaked by a once-in-10,000-years rainfall event.” Alternatively, the statement can be relaxed (and rendered less cluttered and easier to read) by converting the phrasal adjective to a modifying phrase that follows “rainfall event”: “In April 2016, the Houston area was soaked by a rainfall event of the kind that occurs perhaps once in 10,000 years.”
2. In last year’s survey, 43 percent of 40-49 year-olds reported using the bank’s app.
Here, as often, an attempt at suspensive hyphenation, in which one or more words is elided when two equivalent terms can share a supporting word or phrase common to them, has gone awry. The full version of the descriptive phrase is “40-year-olds to 49-year-olds,” and the omission of the first instance of “year-olds” should result in the following rendering: “In last year’s survey, 43 percent of 40- to 49-year-olds reported using the bank’s app.” (If a publications style dictates spelled-out numbers, the correct treatment is “In last year’s survey, 43 percent of forty- to forty-nine-year-olds reported using the bank’s app.”)
3. We expect to complete the project within the next five-to-ten years.
The number range in this sentence is incorrectly styled due to a writer’s mistaken belief that because a range is involved, one or more hyphens belong in there somewhere. What is required, technically, is an en dash (–) rather than a hyphen (-)—but only if the numbers are treated as numerals: “We expect to complete the project within the next 5–10 years.” (Some publications, including many newspapers, dispense with the en dash and use a hyphen in such cases, but most books and magazines employ it; usage online and in other print media varies.) When the numbers are spelled out, no connective symbols are required: “We expect to complete the project within the next five to ten years.”