The comparative adjectives fewer and less are often confused. In general, fewer is used for countable things (like minutes, books, words, etc.), while less is used for non-countable things (like water, sand, etc.). However, there are some common exceptions.

The phrase less than is used for explicitly mathematical comparisons, even when a countable noun is being referred to. (The noun in the following sentence is “ways”, and therefore the comparative adjective should technically be fewer.)

There are 981, or slightly less than 2×10E77, possible ways to fill out a 9×9 sudoku grid.

Another common exception occurs when discussing time. For instance, in the sentence “I will be there in less than five minutes,” the “five minutes” is considered to be a distinct unit of time, even though ‘minute’ is a countable noun. But in other cases regarding time, fewer is clearly the correct word.

It will take fewer years to implement pollution controls than thought.

[Some cases are simply ambiguous, if the noun is neither clearly countable nor non-countable.]