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Bunuel
At a dance there are three married couples: Ann and Andy, Betty and Boris, and Danielle and Dan. The wives select a husband at random with whom to share a dance. What is the probability that each of the three men dances with a woman other than his spouse.

A. 1/9
B. 2/9
C. 1/3
D. 1/2
E. 2/3

Let the proper ordering for the selection of husbands = ABD.

Number of ways to arrange the 3 letters A, B and D = 3! = 6
Of these 6 possible arrangements, only two have no husband in the correct position:
DAB
BDA

Thus:
P(no husband dances with his spouse) = 2/6 = 1/3

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Bunuel
At a dance there are three married couples: Ann and Andy, Betty and Boris, and Danielle and Dan. The wives select a husband at random with whom to share a dance. What is the probability that each of the three men dances with a woman other than his spouse.

A. 1/9
B. 2/9
C. 1/3
D. 1/2
E. 2/3

Formula approach:

Let the correct ordering for the selection of husbands = ABD.
Number of ways to arrange the 3 husbands = 3! = 6

A DERANGEMENT is a permutation in which NO element is in the correct position.
Given n elements (where n>1):
Number of possible derangements = \(n! * (\frac{1}{2!} - \frac{1}{3!} + \frac{1}{4!} +\) ... + \(\frac{(-1)^n}{n!})\)

Thus, given 3 letters A, B and D:
Number of possible derangements = \(3! * (\frac{1}{2!} - \frac{1}{3!}) = 3-1 = 2\\
\)

Since there are 6 possible arrangements for the 3 husbands A, B and D -- and 2 possible derangements -- P(no husband is in the correct position) = 2/6 = 1/3

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