Centimeter is the unit of interest rate. For the annual profit, one percent is one percent, and for the monthly profit, one percent is one thousandth.

So the monthly interest rate is 9%, one month's interest rate is nine thousandths, and one year is.

9/ 1000* 12=0. 108* 100%= 10.8%。

So the annual interest rate is 10.8%.

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The form of interest is very clear, and its surface function is easy for us to understand. But just like the price, why is it a specific value and not another value? What is puzzling is how to decide.

Interest (year) = principal × annual interest rate (percentage) × deposit period

Or interest = principal × interest rate× time