If you use the 20% off coupon your total will be $16.80.
+ Purchase Price = $21
- Discount (21 x 20)/100 = $4.20
= Final Price 21 - 4.20 = $16.80
If you use the $5 dollars off coupon your total will be $16.00.
$21 - $5 = $16.00
The better coupon to use would be the five dollars off coupon because $5 dollars off will be 80 cents less than using the 20% coupon.