Reverse percentage formulas
Original before an increase = Final ÷ (1 + % ÷ 100)
Original before a decrease = Final ÷ (1 − % ÷ 100)
Example: a bill of ₹1,180 including 18% GST had a base price of 1,180 ÷ 1.18 = ₹1,000. A sale price of ₹1,200 after 25% off had an original price of 1,200 ÷ 0.75 = ₹1,600.
Why you can't just subtract the percentage
Taking 18% off ₹1,180 gives ₹967.60, not ₹1,000, because the 18% was calculated on the smaller base price. Always divide by the multiplier instead.
Frequently asked questions
How do I remove GST from a total?
Divide the total by (1 + GST rate ÷ 100). For 18% GST divide by 1.18; for 5% divide by 1.05; for 12% divide by 1.12.
How do I find the price before a discount?
Divide the sale price by (1 − discount ÷ 100). ₹900 after 10% off was ₹1,000.