Given:
Purchase Price: 950
Sales tax: 5% of 950 = 47.50
Total amount applied for credit: 950 + 47.50 = 997.50
APR = 12.5%
Monthly interest rate: 12.5% / 12 = 1.0417%

Interest = 997.50 x 1.0417% = 10.3910 or 10.39

*I assumed that the sales tax paid was included in the credit card payment. Thus, interest for said tax was also computed. The late fee charged was not part of my computation because it is not a part of the original amount.