In a room filled with 7 people

[jonizatt]

In a room filled with 7 people, 4 people have exactly 1 sibling in the room and 3 people have exactly 2 siblings in the room. If two individuals are selected from the room at random, what is the probability that those two individuals are NOT siblings?

a 5/21
b 3/7
c 4/7
d 5/7
e 16/21

In the solution it says that 4 people have 1 sibling, and this would account for 2 sibling relationships. (I don't understand this logic)

It also says that you multiply (7*6)/2 to find that there are 21 different ways to choose two people from the room. I don't understand where they got these #'s.

A more detailed explanation would help out a lot.

Thanks

[StaceyKoprince]

Please search before you post - then you don't have to wait for a reply if the problem has already been posted and discussed! This problem has been posted multiple times before; here's the thread with the largest number of replies;

siblings-problem-combinatorics-t9048.html?hilit=In%20a%20room%20filled%20with%207%20people

If you have additional questions, post in the existing thread. Thanks!