Algebra – Finding Original Price from a 10% Discount

Question

I have a database of prices that already have a 10% discount.

For example a product could be $100 after a 10% discount. Is there a reusable formula I can use to determine what the original price was of all the 10% discounted prices in the database?

Edit: Thank you for the fast responses. Is there any way to account for rounding errors? A real example is a product with a discounted price of \$129.00 Using the X/.9 formula, I get \$143.33 as the original price, which does not actually work out. To have had \$129.00 as the discount price, the original price would have needed to have been \$143.34.

Answer 1

More general:

• Let $d$ denote the discount in %
• let $p_{orig}$ be the original price
• and let $p_{disc}$ denote the discount price

Then it holds

$$\left(\frac{100-d}{100}\right)\cdot p_{orig} = p_{disc}$$

With this formula you are able to calculate the prices even if there is another discount offered.

In your special case you have $d=10$ thus the formula leeds to:

$$\left(\frac{100-10}{100}\right)\cdot p_{orig} = p_{disc}$$

$$\Longrightarrow 0.9\cdot p_{orig} = p_{disc}$$