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# arrangements and combinations watch

1. Hi, the following question has got me.
7 people are to be seated in a row. Calculate the number of ways in which this can be done so that 2 particular people A and B always have exactly one of the others between them (note the answer from the book is 1200)
I just dont know how to get this.
any help greatly appreatiated
2. Consider A - B - - - -,. How many ways can this be done?
Then - A -B - - - etc.
3. and don't forget B _ A _ _ _ _, _ B _ A _ _ _, etc.
4. Think of A?B as one unit, then you 5 things to arrange ... do you know how many ways there are for this

Then double because of B?A

Then x5 because ? could be 5 different people
5. (Original post by TenOfThem)
Think of A?B as one unit, then you 5 things to arrange ... do you know how many ways there are for this

Then double because of B?A

Then x5 because ? could be 5 different people
Thanks 10 of them , yours was the best one .
6. (Original post by BabyMaths)
and don't forget B _ A _ _ _ _, _ B _ A _ _ _, etc.
Thanks baby maths - great help
7. (Original post by Matureb)
Consider A - B - - - -,. How many ways can this be done?
Then - A -B - - - etc.
Thanks for the help matureb

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