Coding-Decoding | Class 6 reasoning — JEE Foundation Syllabus

What you'll learn

Decode a coded word when letters are shifted forward or backward in the alphabet
Use the reverse alphabet table to decode words
Solve number-based coding problems
Interpret symbol codes and apply the same logic to new words

Key concepts

What is Coding and Decoding?

A code is a system where letters, numbers, or symbols are substituted for the real letters of a word. Your job is to find the rule used to create the code, then apply it to a new word.

Always find the rule first. Never guess the answer without identifying the pattern.

Type 1 — Letter Shift Cipher

Each letter in the word is replaced by a letter a fixed number of positions ahead or behind in the alphabet.

Alphabet positions:

A=1  B=2  C=3  D=4  E=5  F=6  G=7  H=8  I=9  J=10
K=11 L=12 M=13 N=14 O=15 P=16 Q=17 R=18 S=19 T=20
U=21 V=22 W=23 X=24 Y=25 Z=26

Worked Example — +2 Shift: If CAT is coded as ECV:

C(3) → E(5) — shifted +2
A(1) → C(3) — shifted +2
T(20) → V(22) — shifted +2

Now decode: ECV → C A T ✓

Worked Example — −3 Shift: If ROPE is coded as OLMB:

R(18) → O(15) — shifted −3
O(15) → L(12) — shifted −3
P(16) → M(13) — shifted −3
E(5) → B(2) — shifted −3

Tip: Always check at least 2–3 letters to confirm the shift is consistent before applying it.

Type 2 — Reverse Alphabet Coding

Each letter is replaced by its mirror-image letter. A↔Z, B↔Y, C↔X, and so on.

Original	A	B	C	D	E	F	G	H	I	J	K	L	M
Code	Z	Y	X	W	V	U	T	S	R	Q	P	O	N

Original	N	O	P	Q	R	S	T	U	V	W	X	Y	Z
Code	M	L	K	J	I	H	G	F	E	D	C	B	A

Formula: Position of coded letter = 27 − position of original letter

Worked Example: Code COLD using reverse alphabet:

C(3) → X(24) | O(15) → L(12) | L(12) → O(15) | D(4) → W(23)
COLD → XOLW

Decode HSVV → S(8→19=S), V(22→5=E), V(22→5=E), T... try: H→S, S→H, V→E, V→E → SHEE? Re-check: H=8, 27−8=19=S; S=19, 27−19=8=H; V=22, 27−22=5=E; V=22→E → SHEE ✓

Type 3 — Number Coding

Letters are replaced by their position numbers (or a modified version).

Rule	Example
Direct position	A=1, B=2 ... Z=26
Reverse position	A=26, B=25 ... Z=1
Position + fixed number	A=3, B=4, C=5 (each +2)

Worked Example: If DOG = 4 15 7, decode: 2 5 4 = ?

2=B, 5=E, 4=D → BED

Worked Example 2: If PEN is coded as 16 5 14, and the rule is direct position: Decode 19 15 14 → S O N → SON

Type 4 — Symbol Coding

Symbols replace letters according to a given key.

Approach:

Use the given word-code pair to build a symbol-letter key.
Only map letters that appear in the given words.
Apply the key to the new word.

Worked Example: Given: FIRE = ★ ✦ ✿ ✪ and ICE = ✦ ✾ ✪ From FIRE: F=★, I=✦, R=✿, E=✪ From ICE: I=✦(confirmed), C=✾, E=✪(confirmed) Code RICE = ✿ ✦ ✾ ✪

Tip: Always confirm a symbol with at least two given words if possible. Never guess a symbol that only appears once.

Step-by-Step Method

Write the given word and its code side by side, letter by letter.
Find the pattern (shift, reverse, position, symbol).
Apply the same pattern to the new word.
Check your answer by reversing the process.

Quick check

If COLD is coded as FROG using a letter shift, find the shift and code WARM.
Using reverse alphabet, what is the code for SUN?
If CAT = 3 1 20, what is DOG in the same system?
If MANGO = 13 1 14 7 15, decode 19 20 1 18.
A letter is coded as the one that comes 4 places before it. If the code is OKNK, what is the original word?

Open the Practice tab for graded questions on Coding and Decoding.