We do it with an example, imagine there is a t-shirt in a store which price is $50 and it is said that it has implied a 15% tax in its price.
So we have a total price of $50 that includes 15% plus of its original price, that is, we do not know how much the 100% or the original price was befotre the tax was added.
We can find that price using a rule of three, simple with direct proportion, consider that the $50 is the 115% of the t-shirt (100% original t-shirt plus 15% tax), the question is to find the 100%:
115% -------> $50
100% --------> x
x = (100)(50)/115
x = 43.48
therefore the original price of the t-shirt, that is the 100% without taxes is $43.48