Since the glasses are 30% off, Garbriela will pay 100% - 30% of the original price, or 70%.
Therefore, Gabriela will pay $72 * 0.7 = $50.40.
The value of the discount may be calculated instead. Moving the decimal one place to the left would quickly yield the value of a 10% discount, $7.20. A 30% discount is three times that, or $21.60. Then we subtract this value from the original price of $72.
$72.00 - $21.60 = $50.40
The final price Gabriela pays is $50.40.