Problem 2a.
The population is the set of everyone at the school. The sample is the set of 15 people who are surveyed.
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Problem 2b.
The sample is supposed to represent the population. It estimates how the population is set up. However, the 15 students surveyed in this manner will all likely play a musical instrument, since they are in band class. Effectively, the results of this survey indicate "100% of the population play a musical instrument", but it's more realistic that there's a sizeable portion of people who don't play a musical instrument. It would be better to randomly pick names from the list of names of everyone at school, and not just band class students.
In short, this sample is not reasonable. It heavily overrepresents those who play instruments, while completely underrepresenting/ignoring those who do not play musical instruments.
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Problem 3.
We use a sample here. The population of all seventh graders in Tallahassee is very large and it would be very costly (in terms of time and money) to get the height of every student. The sample is a smaller group that represents the population at large. In this case, the population is the set of all seventh-graders in Tallahassee.
Answer: Sample
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Problem 4
Since the class is small, and there aren't that many students, you can effectively ask everyone and you are surveying the population. In this case, all you care about is the favorite types of music of people in the class. So that's why the population consists of these people only (and no one else outside the class).
If you want to extend beyond the classroom walls, then the population would get out of hand, and you'd need to take a sample instead.
Answer: Population
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Problem 5
This is similar to problem 3. There are way too many students in Florida, and you'd have to probably narrow things down a bit. Do you mean high school students? Do you mean college students? Perhaps both groups?
The population would be all students defined on how you want to phrase your question as mentioned above. The sample is the small group in which you survey (eg: 200 students).
Answer: Sample