MRBEE
Apr 2, 2008, 04:10 PM
Three married couples are to be randomly seated around a circular table. What is the probability that at least one of the couples will be seated next to one another?
galactus
Apr 2, 2008, 06:16 PM
When seating in a circular permutations, there are (n-1)! Ways to do so without restriction.
In this case, 5!=120 ways to seat the 3 couples(6 people) without restriction.
The opposite of 'at least one' is none. So, find the probability of no couples sitting together
and subtract from 1. Or find the number of ways 1 couple can sit together, 2 couples, and
three couples can sit together and add them up.
For two couples together, we can choose 2 pairs in C(3,2)=3 ways and this pair can be
arranged in 2 ways. The next pair in 2 ways and the next pair in two ways.
So, we have 3\cdot{2^{3}}=24 ways for two couples to be seated together.
Following the same logic, there are 3\cdot{2^{4}} ways for 1 couple to be
seated together and 2^{4}=16 ways for all three to be seated together.
Therefore, we have 16+24+48=88 ways for at least one couple to be seated together.
The probability would be 88/120=11/15.