If the increased or decreased final value of a number is known, it is the method of finding its original value before the percentage operation.

Divide the price including VAT by (1 + VAT Rate/100). For example, a product costing 120 with 20% VAT: 120 / 1.20 = 100.

Divide the discounted price by (1 - Discount Rate/100). For example, a product costing 90 with a 10% discount: 90 / 0.90 = 100.