Less vs. Fewer
Use less for uncountable or mass nouns.
Use fewer for nouns that you can count.
Less
Use less for quantities that are measured rather than counted:
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Less time
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Less effort
You can’t count effort or time (without using units like minutes or hours), so use less.
However, there are exceptions:
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Use less for distances (less than 3 kilometers)
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Use less for sums of money (less than 50 dollars)
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Use less for units of time and weight (less than 7 years, less than 10 pounds)
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Use less for statistics (less than 2,000 people)