s = sale price (in dollars)
p = sticker price (in dollars)
Note: the sticker price is what the customer pays after the discount
Another note: This is ignoring any taxes or other fees
We're told that the sale price is the result of taking 10% off the sticker price. This means...
sale price = (sticker price) - (10% of sticker price)
s = p - 0.10*p
s = 1*p - 0.10*p
s = (1 - 0.10)*p
s = (0.90)*p
s = 0.90*p
So the expression for the sale price is 0.90*p where p is the sticker price in dollars.
Note: this means that the sale price is 90% of the sticker price.
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Now plug in the sticker price p = 80 to find the sale price s
s = 0.90*p
s = 0.90*80
s = 72
The sale price is $72 which is what the customer will pay (if we ignore any taxes or other fees).