Answer:
Sale price of phone: $640
Sale price including tax: $678.40
Explanation:
If the discount is 20%, the price actually paid is (100% - 20%), or 80% of full price.
Multiplying $800 by 0.80 results in the sale price: $640.
The 6% sales tax is 0.06($640), or $38.40.
Paying $38.40 sales tax on the $640 purchase results in a total paid of $678.40.