Coding-Decoding
Subject: Mental Ability | Frequency: 2-3 questions per APPSC paper | Time: 30-45 sec/question
Introduction
In coding-decoding, letters/words are assigned codes following a rule. You must identify the rule and apply it. The EJOTY rule (E=5, J=10, O=15, T=20, Y=25) is the single most useful shortcut.
Core Method
- Assign position values — A=1, B=2... Z=26. Use EJOTY anchors.
- Compare original with code letter by letter
- Find the shift/pattern — calculate position differences
- Verify consistency across all letters
- Apply to target word
- Reverse value: 27 − position (A↔Z, B↔Y, C↔X)
Types
| Type | Rule | Example |
|---|---|---|
| Letter shifting (±n) | Each letter shifted by fixed amount | COME → FRPH (+3) |
| Letter reversal | 27 − position | BAD → YZW |
| Number coding | Letters → position numbers | CAT = 3+1+20 = 24 |
| Symbol substitution | Words replaced by other words | "sky" means "blue" |
| Variable shift | +1, +2, +3 per position | Different shift per letter |
| Sentence coding | Compare multiple sentences | Find common codes |
| Matrix coding | Row-column from grid | M = Row 3, Col 3 = "33" |
Worked Examples — Easy
Q1: APPLE coded as DSSOH → each letter +3. MANGO = PDQJR
Q2: CAT = 24 (3+1+20). DOG = 4+15+7 = 26
Q3: JUNE → HSLC → each letter −2. MARCH = KYPAF
Worked Examples — Medium
Q4: MOUSE → PRXVH → +3. TIGER = WLJHU
Q5: Sentence coding: "go come back" = "ja na pa", "come at home" = "ta ka na", "go to home" = "sa pa ka". Code for "come" = na (common in sentences 1 and 2)
Worked Examples — Hard
Q6: A=2, B=4, C=6... (position × 2). FACE = 12+2+6+10 = 30
Q7: Matrix: A=11, G=22, M=33, I=24, C=13. MAGIC = 33 11 22 24 13
Shortcuts & Tricks
| Shortcut | Details |
|---|---|
| EJOTY | E=5, J=10, O=15, T=20, Y=25 |
| Reverse = 27−n | A↔Z, B↔Y, C↔X |
| Check ±1 to ±5 first | Most common shifts are small |
| Sentence coding | Find common words across sentences |
| Vowel/consonant split | Some codes treat vowels differently |
Common Mistakes
- Miscounting positions (especially K=11 through Z=26)
- Forgetting wrap-around (Z+1=A, A−1=Z)
- Assuming uniform shift — check each letter
- Confusing encoding vs decoding direction
- Ignoring word reversal before shifting
Practice Questions
- If FRIEND = HUMJTF, code for CANDLE = EDRIRL (shifts: +2,+3,+4,+5,+6,+7)
- If BAG = 2+1+7 = 10, then CAR = 3+1+18 = 22
- Reverse code of HELLO = SVOOL (27−n for each)
- MOUSE coded as PRXVH (+3). Code for HOUSE = KRXVH
- If "red blue green" = "pa ta na" and "blue white red" = "sa ta pa", code for "green" = na