1. Problem: Find the next letter in the sequence A Z B Y C X D. 2. Explanation: The sequence alternates between letters starting from A going forward (A, B, C, D, ...) and letters starting from Z going backward (Z, Y, X, ...). 3. Formula: For the nth letter in the forward sequence, use $\text{Forward}(n) = \text{chr}(65 + n - 1)$ where 65 is ASCII for A. For the nth letter in the backward sequence, use $\text{Backward}(n) = \text{chr}(90 - (n - 1))$ where 90 is ASCII for Z. 4. Work: The sequence pairs are (A,Z), (B,Y), (C,X), (D,?). The next backward letter after X (which is 24th letter) is W (23rd letter). 5. Answer: The next letter is W. This method allows you to find the nth term by determining if n is odd or even and applying the corresponding formula.