Let us assume the cost of the stereo to be = x Percentage of sales tax paid by Amanda = 7% Amount of sales tax paid by Amanda = $35 Then 7x/100 = 35 7x = 35 * 100 7x = 3500 x = 3500/7 = 500 dollars So the cost of the stereo that Amanda bought was $500. I hope the procedure is clear enough for you to understand. Using this procedure, you can solve similar problems without requiring any help from outside.