How do I start from a 10% discount and find the original price? 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.
 A: Divide by $0.9$. To check that this works, observe that e.g. $\frac{180}{0.9}=200$, so the original price belonging to a $180\$$ reduced price is $200\$$. (the divisor $0.9$ is just a $10\%$ "reduced" $1$).
A: Let say the price of something is $x$. Giving a discount of 10% means that you decrease the price to 90% of the original. That means that after taking 10% off, you have 90% of $x$ left. That is $0.90x$. 
In your concrete problem you know that $0.90x = 100$.
And you just need to solve this equation to find the original price $x$.
A: Original price $=\frac{10}{9}\times$ Discounted price.
A: Remember that a 10% discount means that it costs 90% of the original price. So you multiply the original price by 0.90. 
Let $O$ be the original price, and $N$ the new price.
So $0.9\cdot O = N$
So $O = \frac{N}{0.9}$
A: As pointed out in another answer
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
$$\frac{100-d}{100}\cdot p_{orig} = p_{disc}$$
So  we can rearrange this to get 
$$p_{orig} = p_{disc} \cdot \frac{100}{100-d}$$
Which lets you find the original price as required. 
A: 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:
$$\left(\frac{100-10}{100}\right)\cdot p_{orig} = p_{disc}$$

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

A: Thinking like this may help you make your own formula,
If you have a Certain percentage (say 90) of the original price you could divide that by 90 to obtain 1% of the original price.  Knowing 1% of the original price it will be easier to find all (100%) of the original price.
