Answer:
$2.21
Explanation:
You want to know the savings between using a $10 off coupon and taking a 15% discount, if the tax rate is 10.3%.
Coupon
The price with the coupon is ...
$79.99 -10.00 = $69.99
When tax is added, the total becomes ...
$69.99 × 1.103 ≈ $77.20
Discount
The price with the discount is ...
$79.99 × (1 -0.15) ≈ $67.99
When tax is added, the total becomes ...
$67.99 × 1.103 ≈ $74.99
Savings
The savings by making use of the discount is ...
$77.20 -74.99 = $2.21
The total savings between the coupon and the discount is $2.21. That is, you save $2.21 more by using the discount instead of the coupon.
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Additional comment
The question seems a bit ambiguous. The word "or" suggests you cannot use both discounts. If you do, the total amount saved will depend on the order in which they are applied.
You get the best result by taking the $10 off the sale price. Doing that gives you a total savings of about $24.27 off the normal price with tax.