Here's my view. Say I have a product with my cost of $100.00

I want to add 28% to that. I think I should end up at $128.00.

My math is simply $10.00*1.28 = $12.80

However, I have had someone else tell me that I'm wrong and need to do
the following.

First. 100-x=y
Second. 100/y=z
Third. A*z=$$$.$$

So,
First. 100-28 = 72
Second. 100/72 = 1.38888889
Third. 10*1.3889 = $13.89


Now to me this person is crazy. I mean I sold stuff for years and
sales tax wasn't that complicated. If something was $10.00 + %5.75tax,
the total is $10.58.

Not 100-5.75 = 94.25, 100/94.25 = 1.061007, $10.00*1.061007 = $10.61

Why would he think that he's correct? Is it some accounting practice,
but not real world practice? Or maybe something a person not originally
from the US would have learned?

Thanks for clearing this up.

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