Coding–Decoding

Before you read the question, write A to Z with their numbers along the top of your rough sheet. Every minute you spend counting letters later is a minute lost.

Two aids repay the effort: the positions A=1, E=5, J=10, O=15, T=20, Y=25, Z=26 (remembered as EJOTY), and the opposite-letter pairing A–Z, B–Y, C–X, D–W, where the two positions always add to 27.

The four types:

1. Letter shifting. Each letter moves a fixed number of steps: FRIEND → GSJFOE is +1 on every letter. Check the shift on two letters before applying it to the rest.
2. Reverse or mirror coding. The word is written backwards, or each letter is replaced by its opposite letter. If a plain shift fails, test the opposite-letter rule next.
3. Number coding. A word is given a number, usually the sum or the product of the letter positions, or a count of the letters.
4. Substitution coding. "If sky is called sea, and sea is called water…" — here nothing is encoded at all; you simply follow the chain of renaming to the final question.

The single most common mistake is in sentence coding, where several words map to several codes: candidates assume the code words stand in the same order as the original words. They usually do not. Match by the word that appears in two sentences and the code that appears in both, and ignore position entirely.

The second most common mistake is miscounting near the end of the alphabet under time pressure — which is exactly what the written-out alphabet prevents.

Write A–Z with positions before starting · EJOTY: A=1, E=5, J=10, O=15, T=20, Y=25 · opposite letters add to 27 (A–Z, B–Y, C–X) · test the shift on two letters first · in sentence coding match the common word to the common code, never by position

If FRIEND is coded as GSJFOE, how is CANDLE coded? → Compare the first pair: F is the 6th letter and G is the 7th, so the shift is +1; check a second pair to be safe — D is 4th and E is 5th, again +1. Apply it: C→D, A→B, N→O, D→E, L→M, E→F, giving DBOEMF. Sanity check: verify the last letter independently — E is 5th, so E+1 is F, and the code does end in F.