Coding questions replace letters with other letters or numbers by a rule. The most common rule is a fixed shift: if CAT becomes DBU, every letter has moved forward by one.
The second family uses position values, so CAT becomes 3-1-20. Learn the alphabet positions in both directions, since some codes count backwards from Z.
A third type reverses the word or swaps letters in pairs. When a code makes no sense, write the word and its code one above the other and mark the movement of each letter.