Find the original number before a percentage increase or decrease was applied.
If a value was increased by P%, the final equals original × (1 + P/100). To reverse this, divide the final by that factor. Similarly, for a decrease divide by (1 − P/100).
Reverse Percentage (increase)
Original = Final / (1 + P/100)
Reverse Percentage (decrease)
Original = Final / (1 − P/100)
Because the percentage was applied to the original, not the final. For example, 20% of 100 is 20, giving 120. But 20% of 120 is 24, not 20. You must divide, not subtract.
Finding the pre-tax price: if an item costs $120 including 20% VAT, the pre-tax price is 120 / 1.20 = $100.