How many 15-letter sequences are there which use only A, B, C and D (and not necessarily all of those), where:
A is never immediately followed by B, B is never immediately followed by C, C is never immediately followed by D and D is never immediately followed by A?

(In reply to AAAAAAAAAAAAAAA (spoiler) by Steve Herman)

Ty, SH. I confirm having hyperlinked your methodology in the official solution.

Posted by K Sengupta on 2024-04-25 08:28:26