When expressing numbers in writing, take care to avoid erroneous styling of number ranges, mixed fractions, and multiple references to categorically similar numbers. The following examples, each of which is followed by a discussion and a revision, exemplify these three categories of style issues.
1. He said there are between 10-15 billion such devices online.
Three ways to express a number range are “from (first number) to (second number),” “between (first number) and (second number), and “(first number)–(second number).” The third choice features an en dash, though some publications, for the sake of simplicity, use a hyphen, and many writers do so because they are unaware of the distinction. More significantly, that option is appropriate only for numerals, while the first and second choices apply to both numerals and spelled-out numbers.
However, do not mix from or between with an en dash (or a hyphen) when expressing a number range; to correctly format the given sentence, choose from among the following treatments: “He said there are from 10 to 15 billion such devices online,” “He said there are between 10 and 15 billion such devices online,” and “He said there are 10–15 billion such devices online.”
2. The pitchers combined for 32/3 shutout innings.
When, in a mixed fraction, the fraction is set as a case fraction (with small upper and lower numbers separated by a narrow horizontal line), the letter space is unnecessary, but for clarity, it is essential when the fraction is expressed vertically, as here: “The pitchers combined for 3 2/3 shutout innings.”
3. The process should be documented over a six- to 24-month period.
The Associated Press Style Book recommends spelling out numbers up to nine and (with some exceptions) using numerals for larger numbers, even when two or more numbers in each category appear in proximity in a reference to related amounts or values.
However, Chicago puts the breaking point at more than one hundred, so in this example, both numbers would be spelled out: “The process should be documented over a six- to twenty-four-month period.” (But if the numbers are unrelated, as in the following sentence, there is no need for consistency: “Even as recently as 200 years ago, a fifty-mile journey was a major undertaking.”)
(Both style manuals are helpful to writers, but I favor Chicago as a more comprehensive resource that encourages a more sophisticated approach to writing than the AP Style Book, which models a simple, more functional prose style.)