You use per cent to talk about amounts. For example, if an amount is 10 per cent of a larger amount, it is equal to 10 hundredths of the larger amount. 20 to 40 per cent of the voters are undecided We aim to increase sales by 10 per cent Per cent is also an adjective. There has been a ten per cent increase in the number of new students arriving at polytechnics this year. Per cent is also an adverb. its prediction that house prices will fall 5 per cent over the year