According to the interest rate conversion formula A= 12×M, where a is the annual interest rate and m is the monthly interest rate. Substituting the monthly interest rate of 0.23% into the formula, the annual interest rate is A= 12×0.23%=2.76%. So the annual interest rate of 0.23% is 2.76%. This is calculated according to the interest rate conversion formula to reveal the mathematical relationship between monthly interest rate and annual interest rate.