Solving the hardest question of a British Mathematical Olympiad

Thanks to Nahian for the suggestion! This is a difficult factorial problem.

British Mathematical Olympiad 2002-2003 problem 5
https://bmos.ukmt.org.uk/home/bmo1-20...
A Mathematical Olympiad Primer Paperback – 1 Aug. 2011
https://www.amazon.co.uk/Mathematical...
Math Forum post
http://mathforum.org/library/drmath/v...
Math StackExchange post
https://math.stackexchange.com/questi...

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