From here...
http://oxforddictionaries.com/words/less-or-fewer
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'Less' or 'fewer'?
'Less' or 'fewer'?
People often don’t know when to use less and when to use fewer in a sentence. Here’s how to get it right.
Use fewer if you’re referring to people or things in the plural (e.g. houses, newspapers, dogs, students, children). For example:
People these days are buying fewer newspapers.
Fewer students are opting to study science-related subjects.
Fewer than thirty children each year develop the disease.
Use less when you’re referring to something that can’t be counted or doesn’t have a plural (e.g. money, air, time, music, rain). For example:
It’s a better job but they pay you less money.
People want to spend less time in traffic jams.
Ironically, when I’m on tour, I listen to less music.
Less is also used with numbers when they are on their own and with expressions of measurement or time, e.g.:
His weight fell from 18 stone to less than 12.
Their marriage lasted less than two years.
Heath Square is less than four miles away from Dublin city centre