National debt means that the government borrows money from ordinary people and agrees to return it to them within a certain period of time, paying interest higher than or close to time deposits, but the income from national debt is tax-free. There are two kinds of national debt, one is called book-entry national debt, which is usually bought and sold in the securities market and the counter of state-owned banks; The other is voucher-type national debt, which has poor liquidity. People can't transfer them when they buy them, but they must hold them until they get full interest at maturity. If they are in urgent need of cash, they can only exchange it with the bank at a very low interest rate and pay a handling fee of one thousandth at the same time, or they can mortgage the national debt to the bank for a loan, the loan amount is about 90% of the national debt you hold.

If the interest rate is 4.3%, the sum of principal and interest after 10 is: principal *( 1+4.3%* 10).

The simple interest formula is: principal *( 1+ interest rate * year)