If you would like to know where Guillmero will get the better price, you can calculate this using the following steps:
a coupon: $49.99 - $10 = $39.99
Costmart: 25% of $49.99 = 25% * 49.99 = 25/100 * 49.99 = $12.4975 = $12.50
$49.99 - $12.50 = $37.49
Guillmero will get a better price at Costmart.