Since the tip was 20% of the amount including tax, we don't need to know the tax rate to figure the total amount.
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The bill before tip was $3.50/0.20 = $17.50, so the total amount including tax and tip was $17.50 +3.50 = $21.00.
(Tax rate doesn't figure in, unless we want to know the bill before tax was added. As it works out, these numbers don't add up. The bill <em>with tax</em> would be either 17.49 or 17.51. It cannot be 17.50.)
