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I'm not for sure if this is what you meant though sorry
Step-by-step explanation:
115 = 5x + 15
115 - 15 = 5x
100 = 5x
20 = x
The answer to the question
All possibilities = 9!
possibilities that harry sits at an end = 8! + 8!
possibilities that harry doesn't sit at an end = 9! - 8! -8!