Question 1117698: A committee is formed from 3 seniors, 2 juniors, 2 sophomores, and 1 freshmen. All students are seated around a circular table.
If students of the same grade level must sit together, how many different ways are there for the students to be seated?
Answer by ikleyn(52788) (Show Source):
You can put this solution on YOUR website! .
Assume that the seats are all numbered sequentially from 1 to 8 and placed around the circular table in order in clockwise direction.
Next assume that the freshmen occupies the chair #1.
Then you may think, as a first approximation, that each of the three groups (3 seniors / 2 juniors / 2 sophomores) represents one object.
Then you have 3 objects, and there are 3! = 1*2*3 = 6 permutations to order them. (1)
Inside of each group you have 3! = 6 ways to order seniors, (2)
2! = 2 ways to order juniors and (3)
2! = 2 ways to order sophomores, (4)
and these interior orderings are independent.
Hence, the final answer is
(6 permutations of (1)) * (6 permutations of (2) ) * (2 permutations of (3) ) *(2 permutations of (4) ) = 6*6*2*2 = 144.
Answer. There are 144 ways (144 circular permutations) to do it.
Thanks to this single freshmen, who facilitated the solution so much !
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On circular permutations, see the lesson
- Persons sitting around a cicular table
in this site.
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