How It Works
To recover the original amount before a percent markup or discount, divide the final observed value by (1 + p ÷ 100). If p is negative this recovers a pre-discount value.
Original = Final ÷ (1 + p ÷ 100)
Example: A price of 125 after a 25% markup gives original = 125 ÷ 1.25 = 100. If p = −100% the denominator is zero and the original value is undefined.