Now if the cost is $11, you'd have to offer $11. He'd get to keep $9.90 after paying the 10% tax.
But that's only if you base the calculation on the wrong original number. If I pay 11.00, the base price for the calculation is 10.00, not 11. Like I said, that's why they make the base price and the sales tax so clearly separated on the receipt. So you don't make the wrong calculation.
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Tony Fabris