Have you tried Robert Kaplinsky's Lesson called "Which Bed Bath & Beyond Coupon Should You Use?" This lesson is a great way to get students practicing and observing patterns around calculating percent off of an item. In this problem he presents two coupons:
Next, he presents students with a few different items and asks students to identify which coupon would give them the better deal. Through these calculations, students begin to discover patterns around which priced items lend to using which of the two coupons. After practicing these calculations, to make this connection even stronger for students and allow for instant feedback, try turning this problem into a program.
Very cool way to integrate the lesson and coding! Well done!