Uncle Sun saved 20,000 yuan in the bank the year before last, and kept it for two years on a regular basis. After the expiration of this year, * * * will withdraw 20,840 yuan, with an annual interest

Uncle Sun saved 20,000 yuan in the bank the year before last, and kept it for two years on a regular basis. After the expiration of this year, * * * will withdraw 20,840 yuan, with an annual interest rate of%. 20,000 yuan will be deposited in the bank for two years, and 20,840 yuan will be withdrawn with interest at maturity. Its two-year periodic interest is: 20840—20000 = 840; Its interest rate = 840 ÷ 20000 ÷ 2 = 0.025438+0; With a percent sign, the interest rate is 2. 1%.