This is a sales tax problem.

cost of the CD ($) + sales tax amount ($) = total price of the purchase ($)

understand that the sales tax amount depends on the price of the item(s) you're purchasing, in this case, the sales tax amount is 7.5% of $12.00. To find this amount, you first have to convert the sales tax from percent form to decimal form:

7.5% / 100% = 0.075

and so, ($12.00) + ($12.00 * 0.075) = $12.00 + $0.90 = $12.90