This paper introduces a percentage activity based on the context of "Black Friday" discounts. The aim of the study was to reveal the existing and new conceptions of the 7th grade students regarding the topic of percentage calculation. The activity was designed considering the students' prior knowledge in the concept of "Ratio and Proportion" as well as in "Percentages." The participants of the study were 17 seventh grade students attending a state middle school in a province in the Eastern Black Sea region of Turkey during the academic year 2019-2020. The data were collected using a worksheet created by the researchers based on the realistic mathematics education framework. After implementing the activity, a form was completed by the students to find out their views on the activity. As the present study reveals the students' conceptions and mistakes, it offers suggestions for future implementation and development to researchers and teachers.