Question 112
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?
- 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?