The interest calculation method for Bank of China credit cards is: interest = principal * daily interest rate * number of days of use. The upper limit of the interest rate for cash withdrawals and overdue payments on Bank of China credit cards is 0.05% daily. Compound interest is calculated on a monthly basis, and the annualized interest rate is approximately 19.9%. For example, if someone uses a Bank of China credit card to withdraw 5,000 yuan in cash, the usage period is 30 days, and the daily interest rate is 0.05%, then the interest is 5,000*30*0.05%=75 yuan.