= price / (1 - discount)

If you have a discounted price and know the discount percentage, you can calculate the original price with a simple formula that divides the discounted price by the result of 1 minus the discount percentage.

In the example, the active cell contains this formula:

=C6/(1-D6)

In this case, Excel first calculates the result of 1 - the value in D6 (.2) to get 0.8. Next the value in C6 (56), which represents the discounted or "sale" price, is divided by 0.8 to get a final result of 70:

=56/(1-0.2)

=56/0.8

=70