The second volume of Huanggang 360 sixth grade mathematics is to teach students how to calculate interest rates. Money deposited in the bank is called principal. The extra money paid by the bank when withdrawing money is called interest. The ratio of interest to principal per unit time is called interest rate, which is usually expressed as a percentage, that is, the percentage of interest to total principal. The interest rate is determined by the state according to the needs of economic development. It is relatively fixed for a period of time, and all banks must abide by it, but the interest rate will be different in different periods. Calculated by year, it is called annual interest rate, and calculated by month, it is called monthly interest rate. The interest rate varies according to the deposit term and deposit method. Note: Interest = principal × interest rate × time.