Percentages
It's known the current price of an item is $339 after 13% tax and some discount to be determined.
Let's get this done from end to beginning.
The final price is $339 and it has already been charged 13% tax.
If the price before tax was x, then the price after tax is:
x + 13% of x
Calculating:
x + 13/100*x
x + 0.13 x
1.13x
The final price is 113% (100% plus 13% tax) of the previous price.
We can now calculate the value of x:
x = $339 / 1.13
x = $300
This price was already discounted by a certain percent with respect to the original price. The discount was $400 - $300 = $100
The percent discount is $100 / $400 * 100 = 25%
The percent discount is 25%