I'll do the math for you, and I'll give you a score. April 1 day, the interest is 2.60 = 400 * 0.0005 * 13 (from March 19 to March1day, 13 days), and the credit card records simple interest on a daily basis and compound interest on a monthly basis. The interest on May 1 is 3.8=402.6*0.0005* 19 (you repaid the loan on the 20th, so the interest is calculated from April 1 to 19, with five ten thousandths of a day, and two decimal places are reserved, so do the calculation, that's it).