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# Permutations and combinations question watch

1. Im stuck on d). Not sure how to approach this question
2. (Original post by SWISH99)

Im stuck on d). Not sure how to approach this question
Given that C, D E will be sitting somewhere (i.e. they are guaranteed to be picked) and that there are no adjacency requirements, I think you can just concentrate on those three and disregard everybody else. Then, how many ways are there of arranging C, D and E, and out of those, how many have D as the middle letter?
3. (Original post by old_engineer)
Given that C, D E will be sitting somewhere (i.e. they are guaranteed to be picked) and that there are no adjacency requirements, I think you can just concentrate on those three and disregard everybody else. Then, how many ways are there of arranging C, D and E, and out of those, how many have D as the middle letter?
Well there would be 2. CDE and EDC. Im not sure what to do with the rest of the numbers though
4. (Original post by SWISH99)
Well there would be 2. CDE and EDC. Im not sure what to do with the rest of the numbers though
Further to that you will need the number of unrestricted arrangements of C, D and E. Then the probability is one number divided by the other.

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