Although there is no knowledge about interest rate and interest in primary school mathematics textbooks, in real life, most people have to go to the bank to make savings, whether it is current or regular, which is bound to be related to interest rate and interest. Therefore, it is undoubtedly of practical significance to understand the connection and difference between these two concepts and deal with the practical problem of saving in life. When you go to the bank to deposit money, the amount of savings is called "principal", or "principal" for short. According to the amount and time of the deposit, the reward paid by the bank to the depositor is called "interest". The ratio of monthly (or annual) interest to principal is called "interest rate". In other words, the monthly (or annual) interest is calculated according to the principal and interest rate. The monthly interest rate is called the monthly interest rate, and the annual interest rate is called the annual interest rate. Generally speaking, the interest rate is calculated on a monthly basis, usually expressed in the form of one thousandth.