Answer:
He can randomly survey 50 sixth-graders in the school. Because if he asked 30 people in the band some people might be in a different grade, if he randomly 50 boys in the school then what if half didn't even play anything. If he did friend what if most friends don't even play a instrument.
Then which one would it be it would be more easier if he did 50 random sixth-graders because they might know friends or themselves and a person who plays an instrument.
Hope this helps