There are 50 students in a class, 32 of them join the calligraphy group and 28 join the painting group, of which 15 is both studied. How many people didn't join the interest group?

There are 32 people in the calligraphy group and 28 people in the painting group, all of whom have studied 15, indicating that there are 32+28- 15 = 45 people who have participated in interest groups, and 15 is subtracted because this 15 is included in both 32 people and 28 people. The number of people who have never participated in interest groups is 50-45 = 5.

Answer: 50-(32+28- 15)=5 people.