3. when calculating your final total, including sales tax, you must (1 point) multiply the sales tax percentage by the original cost of the purchase (before any discounts are applied) and then add it to that total. multiply the sales tax percentage by the sales cost of the purchase (including any applicable discounts) and then add it to that total. divide the sales tax percentage by the original cost of the purchase (before any discounts are applied) and then add it to that total. multiply the sales tax percentage by the store's cost of the purchase and then add it to that total.