A puzzle based on combinatorics, which is the study of counting discrete structures.

A puzzle based on combinatorics, which is the study of counting discrete structures.

For example, how many ways to arrange $3$ of $abcd$, where order is important?

We have $abc,abd,acd,bcd$ and six of each (for example $abc,acb,bac,bca,cab,cba$), and so the answer is $24$.

For basic combinatoric examples and explanations, see:

Puzzles where permutations, sets, graphs, lattices and other combinatorial mathematics feature prominently should include this tag.