Part A
Let's do this step-by-step. Since she first deducts $10, the total price of the dress is now just $50. Then, she applies the 30% discount which is:
Subtract 15 from 50 to get $35. $35 is the total cost she ends up paying.
Part B
Let's also do this. 30% off of $60 is:
Subtract 18 from 60 to get $42. Then, since she applies the $10 discount, let's subtract $10 from $42 to get $32. She ends up paying $3 less dollars if the cashier puts the 30% off discount before the $10 off discount.