Coding-Decoding — Study Notes

Overview

Coding-Decoding is a pattern-recognition topic in logical reasoning where words, letters or numbers are transformed using a specific rule. Your task is to identify the rule from examples and then apply it to encode a new word or decode a given code. This topic appears in nearly every SOF IMO paper and is a quick-scoring section if you master the common patterns.

In the exam, you'll see 2–4 coding-decoding questions, typically in the first half of the Logical Reasoning section. The good news: once you spot the rule, the answer is straightforward. The challenge is speed—you must decode the pattern in under 60 seconds per question. Most errors come from misreading the rule or skipping a step in multi-layer codes.

Students must master letter-position values (A=1, B=2, ... Z=26), reverse alphabets, arithmetic operations on positions and symbol-substitution patterns. Practice 20–30 varied questions to internalize the common types.

Key Concepts

• **Letter Position Value**: Each letter has a position in the alphabet—A=1, B=2, C=3, ... Z=26. Many codes involve adding or subtracting from these positions.

• **Reverse Alphabet**: The reverse alphabet maps A↔Z, B↔Y, C↔X, etc. A common trick is to replace each letter with its opposite-end partner.

• **Shift/Caesar Cipher**: Move every letter forward or backward by a fixed number of steps (e.g. shift +3 means A→D, B→E, C→F).