How can you estimate the amount of sales tax and the total cost of an item? support your explanation with an example from this lesson.

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