7. (5 points) It's the New Year, and the students are going to make some handicrafts to give to the elderly in the nursing home. At the beginning, the students in the art group work for one day, then 15 more students join them and they work together for two more days, just completing the task. Assuming each student has the same work efficiency, and one student would need 60 days to complete the task alone. How many students are in the art group? $\qquad$