Generally, fewer is used when the number of things is counted (“fewer problems”) whereas less is used when the number is measured (“less trouble” or “less time”). However, this is not a strict rule and there are accepted instances of less being used with countable amounts such as “250 words or less,” “3 items or less,” and especially with money (“less than $20”) and distance (“less than 3 miles”).
fewer가 원래맞고 less가 변칙임 모르면서 우기지 좀 마라