Combinatorics › Permutations and arrangements › Arrangements of letters
Strings with distinctness constraints
High school · ●●○○○ 20 pts
Alec wishes to construct a string of 6 letters using the letters A, C, G, and N, such that: - the first three letters are pairwise distinct, and so are the last three letters; - the first, second, fourth, and fifth letters are pairwise distinct. In how many ways can he construct the string?