About Tier 3 - Dimension
Requires several pieces of reasoning to be organised into a solution.
Difficulty describes the challenge, not the solver. It is a Society guide rather than a formal academic level.
The tier is a Society guide to the amount of reasoning, technical work and insight a problem is expected to require. Your experience may vary depending on the topic and your background.
Every Letter in the Wrong Envelope
Discrete Mathematics / Derangements - 7-10 min
Four letters are addressed to four different people, with one correctly addressed envelope for each person. Put exactly one letter in each envelope. How many assignments place every letter in the wrong envelope?
Count complete assignments, not the different orders in which you could fill the envelopes.
Assumptions
All four letters and all four envelopes are distinct. Exactly one letter goes in each envelope; only the assignment matters, not the order in which letters are inserted.
More Options for Every Letter in the Wrong Envelope
Collection question · Revision 1 · ID: four-derangements