Q. When hours and minutes are mixed in a sentence that is describing a duration, are all numerals used? For instance, is it “The spacewalk lasted 7 hours and 54 minutes” or “The spacewalk lasted seven hours and 54 minutes”?
A. Chicago style allows for mixing numerals and spelled numbers if they describe different categories of objects (like hours and minutes). All the numbers in a given category should be treated the same way. So both your versions are fine, although our preferred rendering would be “The spacewalk lasted seven hours and fifty-four minutes.”