No my dear...math is my thing!
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The $15 tariff cost, is added to the factory cost, (along with some other expenses that for simplicity, I did not mention in my example like freight and insurance costs and any other duties) gives you, as the importer, the LANDED COST of the item you purchased from overseas.
When a whole seller imports a product from overseas instead of buying it domestically from a manufacturer here, the importer calculates what the total cost for the widget would be when imported, the factory cost, the freight cost to ship it here to the USA, the insurance cost you pay to cover a ship's possible mishaps with your container freight shipping, like theft, or water damage from storms, or even sinking...(which happened to me once), and any duties or tariffs you have to pay the port of entry, in order to unload and receive your goods on to a truck to get it to your warehouse distribution center etc...and the item's total landed cost....all those fees paid to get that item you bought from overseas to the warehouse and COMPARE it to what cost of the widget item would be if you paid a domestic manufacturer for the item.
That's how one makes their financial decisions on whether to import or buy domestically if available to do so, you need to know all of what you will spend to get that item bought overseas, here... And compare the gross margin to what you would pay for it domestically.
And that includes any tariffs and other fees to get the total cost for the item to calculate the projected gross margin profit on the item when choosing the selling price for your customers.
(There are many more things considered that are political in nature, and there are market strategies, and the competition, ....that are considered in pricing the product....but it could take weeks to teach all of them....)
The example you gave is wrong because you did not account for the cost of the tariff in to the cost if the item, when pricing the item for sale.
You'd lose your ass if you owned the company, and job if working for an importer as a buyer....if you did something like that!