complete the sentences with fewer or less

Less - сравнительная степень прилагательного little. Используется, главным образом, с неисчисляемыми существительными.

I spend less time performing tasks.

In this glass less water than in that.

Autumn is less rainy this year.

I know less about art than about music.

Fewer - сравнительная степень прилагательного few. Используется с исчисляемыми существительными.

People are buying fewer books these days.

Fewer people today carry cash.

I have fewer friends than my sister.

I collected fewer marks than my brother.