Introduction to Coding-Decoding

Introduction to Coding-Decoding

In coding-decoding questions, a word or number is written in a secret form (a code) using a fixed rule. Find the rule, then apply it to a new word or decode a given code.

Common types of coding

• Letter shifting: each letter moves forward/backward by a fixed number of places (e.g. +1, -2).
• Reverse/opposite: letters are reversed or replaced by their opposite letter (A↔Z, B↔Y).
• Letter-to-number: letters become alphabet positions (A=1 ... Z=26).
• Substitution: one word is called by another name (e.g. 'sky' is called 'water').

Quick tip

Remember EJOTY: E=5, J=10, O=15, T=20, Y=25. Opposite letter positions add up to 27.

Worked example

Question: If CAT is coded as DBU, how is DOG coded?

Solution:

1. Compare letters: C→D (+1), A→B (+1), T→U (+1).
2. Rule: each letter moves one place forward.
3. Apply to DOG: D→E, O→P, G→H.

Answer: DOG is coded as EPH.

Analogy

Analogy: A secret language between friends

Coding-decoding is like two friends inventing a secret language at school:

• They agree on one rule — for example "shift every letter one step forward" (CAT → DBU).
• Anyone who discovers the rule can read every message.

In the exam, your job is to be the detective: compare the given word with its code, find the rule (shift, reverse, number value or substitution), and then apply the same rule to the new word.

Tip: Write A–Z with position numbers (A=1 … Z=26) in the margin — it is your "code book".