Prior to this lesson, I created 12 questions. Each question included a picture of an item from the grocery store and its cost. In addition, I put a picture of a coupon or a picture of sales tax next to the good to be purchased.
Students would scan the QR code to display the question. Once the question was displayed they had to determine if they must add or subtract (coupons--subtract; sales tax--add). Once they determined adding or subtracting they then had to work together to figure out what the final cost of the good would be if they were to pay.
Check out the video below for a quick look at this learning experience. The kids loved it and did great.
If you want to try it yourself, you can get the materials from Teachers Pay Teachers for free.