In a certain code language, ROBUST is coded as QNATRS. In the same language…

In alphabet code-substitution puzzles, every letter of a word is replaced by another letter using ONE fixed positional shift along the alphabet (the same number of places forward or backward for every letter). The shift amount is found by comparing a given word to its coded form letter by letter, then the identical shift is applied to any new word in the same code.

1. Compare ROBUST with its code QNATRS letter by letter: R->Q, O->N, B->A, U->T, S->R, T->S.

2. Each letter of the code is exactly one place earlier in the alphabet than the corresponding letter of the word (a shift of -1), and this holds for all six letter pairs, confirming a single consistent rule.

3. Apply the same -1 shift to each letter of ZXCMP in order: Z, X, C, M, P.

4. Shifting Z, X, C, M, P each one place back in the alphabet gives Y, W, B, L, O respectively.

5. Reading these letters in order gives the code YWBLO.

Cross-check: reversing the same rule (shifting every letter of YWBLO one place forward in the alphabet) must reproduce ZXCMP exactly - Y, W, B, L, O shifted forward by one place each give back Z, X, C, M, P, confirming the derived code is consistent in both directions.