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How many ways can you arrange the nine letters A, B, C, D, E, F, G, H, I in a row, if B cannot immediately follow A?

 May 8, 2020
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The 9 letters can be arranged in: 9! =362,880 ways.

 

Each of the 9 letters can follow another letter in: 1/9 x 9! =40,320  ways.

 

Conversely, each letter will be separated from the other 8 letters in:8/9 x 9! = 322,560 ways - or this is the number of ways where B cannot immediately follow A.

 May 8, 2020

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