There is some interesting material about math symbols at The History of Mathematical Symbols by Douglass Weaver and Anthony D. Smith.
For example, they say:
Percent has been used since the end of the fifteenth century in business problems such as computing interest, profit and loss, and taxes. However, the idea had its origin much earlier. When the Roman emperor Augustus levied a tax on all goods sold at auction, centesima rerum venalium, the rate was 1/100. Other Roman taxes were 1/20 on every freed slave and 1/25 on every slave sold. Without recognising percentages as such, they used fractions easily reduced to hundredths.
In the Middle Ages, as large denominations of money came to be used, 100 became a common base for computation. Italian manuscripts of the fifteenth century contained such expressions as “20 p 100” and “x p cento” to indicate 20 percent and 10 percent. When commercial arithmetics appeared near the end of that century, use of percent was well estasblished. For example, Giorgio Chiarino (1481) used “xx. per .c.” for 20 percent and “viii in x perceto” for 8 to 10 percent. During the sixteenth and seventeenth century, percent was used freely for computing profit and loss and interest. (NCTM p146,147}