Two jobs in one: add sales tax to a price, or take a receipt total and work out what the price was before tax.
Sales tax is charged on the pre-tax price, so the math is: tax = price × rate, and total = price × (1 + rate). Working backwards, the pre-tax price is the total divided by (1 + rate) — not the total minus the same percentage, a common mistake that overstates the tax.
Why is the backwards answer not just total minus 8%? Because the percentage applies to the smaller pre-tax price, not the total. Dividing by 1.08 removes exactly the tax that was added; subtracting 8% of the total removes too much.
What rate should I use? The combined rate where the purchase happens — state plus county and city additions. It is printed on most receipts.