Reverse tip formula
Original bill = final total ÷ (1 + tip rate ÷ 100)
This is correct when the final total consists of a bill plus a percentage tip on that bill. If sales tax, fees, or a service charge are included, they must be handled separately.
Example: A $120 total including a 20% tip means a $100 original bill plus a $20 tip, assuming there is no tax.