1. Interest = interest base × daily interest rate × interest days, interest base = current bill amount-minimum repayment amount, and interest days = days from consumption date to next billing date.

2. For example, suppose that the billing date of a credit card is 1th of each month and the final repayment date is 3th of each month. I spent 5, yuan on January 15th, and by the billing date of February 1th, the current repayment amount is 5, yuan, and the minimum repayment amount is 5 yuan. If only the minimum repayment amount is paid on February 3th, the interest to be paid is: interest =(5-5)×.5%×46=13.5 yuan.