How many different character sequences of length 6 exist in a four-letter alphabet that contain exactly two identical letters.

This task is entirely related to the topic of combinatorics. To solve this, we need a repetition placement formula. The rationale for this lies in the definition of repetition placement.
Repetition placement is used when we are given N items and need to be placed in K positions, while we are allowed to repeat some elements.
Formula for repetition placement: A (n, k) = n ^ k;
Only in our case, this formula works only 2 times, since we can only repeat two elements, then our formula will be: 4 * 4 (since we can select the first two elements in four ways) * 3 * 2 * 1 * 0. It turns out that we are missing characters, that is, it is impossible to make a sequence of 6 characters by repeating exactly two.