Percent (from Latin per centum,"by the hundred") may be spelled as one or two words; 10 percent means "10 out of 100" or "10 in 100." Percent is colloquial when used as a substitute for percentage (the noun). Percentage is colloquial when used in the meaning of "profit" or "advantage," as in "What's the percentage in hard work?"
Dictionary of problem words and expressions. Harry Shaw. 1975.