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Method-2



Let department members are: A, B, C, D, and E and A and B refuse to attend the conference together.

Hence, Total cases in which we A and B can be there in a pair:
    • Both A and B are in the pair OR
    • A is in the pair and B is not in the pair
    • B is in the pair and A is not in the pair
    • Neither of A or B is in the pair.

But, out of all the cases we are not selecting 1 case:- When both A and B are in the pair.

Thus, if we remove the case when both A and B are in the pair from the total pairs, we will get the answer.

    • Total Pairs: \(^5c_2\)
    • Total Pairs when both A and B are in the pair= 1

Thus, required answer= 10-1
Hence, total ways=9

Hence, the correct answer is option B.

Answer: B
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using the formula for k combinations of n, with n=5, k=2:

\(5!/(2!*3!)=10\)

We know that 1 combination out of the 10 possible ones is not allowed, so: 10-1=9

Answer B
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1C1*3C1 + 1C1*3C1+3C2=9

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Total number of pairs = 5C2 = 10
One pair cannot attend together out of 10 = 10 - 1 = 9

Answer: B

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