The price paid for using money is called principal rate interest loan interest applies when interest for each year is based on the amount of the loan or investment. Compound Simple Linear Quadratic interest most often applies to savings accounts, installment loans, and credit cards and is based on the accumulated amount. Compound Simple Linear Quadratic is the amount of money borrowed or invested. Interest Principal Principle Rate 5. is the percent of the principal paid as interest per time period. Principal O Interest O Rate Principle