Final answer:
In a group of 15 people, it is not possible for each person to have exactly 3 friends. However, in a group of 4 people, it is possible for each person to have exactly 3 friends.
Step-by-step explanation:
In a group of 15 people, it is not possible for each person to have exactly 3 friends. Let's assume that each person has exactly 3 friends. Since friendship is a symmetric relationship, let's consider person A, who is friends with person B, person C, and person D. But this means that person B, person C, and person D have already counted their friends and they have 3 friends each. So, when we consider each person in the group, there are no more people left for them to be friends with. Hence, it is not possible for each person to have exactly 3 friends in a group of 15 people.
In a group of 4 people, it is possible for each person to have exactly 3 friends. Let's assume that we have four people named A, B, C, and D. Person A can be friends with all the other three people (B, C, and D), and similarly, each of the other three people can be friends with each other. In this case, each person has exactly 3 friends, making it possible for each person to have exactly 3 friends in a group of 4 people.