Add tax to a price — or pull the tax back out of a tax-included total.
Adding tax is a single step: multiply the price by the tax rate to get the tax, then add it on — $100 at 7.5% is $7.50 tax, for a $107.50 total. Going the other way is the part people miss. To find the pre-tax price hidden inside a tax-included total, you divide rather than subtract: total ÷ (1 + rate). So a $107.50 receipt at 7.5% breaks down to $107.50 ÷ 1.075 = $100 plus $7.50 tax. Subtracting 7.5% of $107.50 would give the wrong answer, because the tax was charged on the smaller pre-tax figure. US sales tax varies by state, county and city, so use your exact local combined rate.