Some writers are confused about when to use “fewer” rather than “less than.” They aren’t interchangeable! Here’s the simple rule:
Fewer is used when referring to something that can be counted: There are fewer students in my class this semester. Dr. Smith gave fewer assignments than Dr. Brown. Students and assignments are discrete things, which can be counted.
Less than or less is used when referring to something that can’t be counted, either because it has to be weighed or because it’s abstract: Put less sugar in my coffee, please. There is less noise in this room. Sugar and noise can’t be counted, only weighed or measured.