The correct answer is (C). After the first 10% discount, the price was $450 ($500 minus 10% of $500). After the second discount, which is calculated based on the $450 price, the price of the stereo is $405 ($450 minus 10% of $450). A 20% tax on $405 is $81. Thus, the customer has paid $405 1 $81 5 $486. A percent-change problem might also involve an accompanying chart or graph, which provides the numbers needed for the calculation.