Let's first determine the total number of the students.
=> 10 freshman
=>12 sophomores
=>15 juniors
=> 30 seniors in the club.
Since the adviser will only chose 1, let's add then divide by 4 to get the average.
=> 10 + 12 +15 + 30 = 67 students
=> 67 / 4 = 16.75
OR close to
=> 15/67