Comprehensive Guide to Coding and Decoding for CSAT (UPSC/MPSC)

1. Introduction and Importance

Coding and Decoding is a highly significant and scoring topic in the Logical Reasoning section of the CSAT for UPSC and MPSC examinations. In these questions, a word, number, or phrase is expressed (coded) in a secret way based on certain hidden rules or patterns. Candidates are required to crack this rule and apply the same logic to decode a given message or code a new word.

The primary reason this topic is heavily featured in aptitude exams is that it tests your logical deduction, observation skills, and ability to grasp hidden patterns rapidly. Mastering this topic guarantees 2-3 accurate answers in the exam in a very short amount of time, boosting your overall CSAT score without complex mathematical calculations.

2. Core Concepts, Formulas, and Tricks

The fundamental requirement for Coding-Decoding is the same as Letter Series: you must know the English alphabet’s positional values forward and backward.

A. Alphabet Positions and Tricks

• Forward Positions: A=1, B=2, C=3 … Z=26.
• Backward Positions: Z=1, Y=2, X=3 … A=26. (Remember: Backward = 27 – Forward).
• The EJOTY Trick: E(5), J(10), O(15), T(20), Y(25).
• Opposite Pairs: Sum of opposite letter positions is always 27.
  Examples to remember: A-Z (Azad), B-Y (Boy), C-X (Crux), D-W (Dew), E-V (Evening), F-U (Full), G-T (GT Road), H-S (High School), I-R (Indian Railway), J-Q (Jungle Queen), K-P (Kanpur), L-O (Love), M-N (Man).

B. Types of Coding and Decoding

• Letter Coding: Letters are replaced by other letters according to a specific rule (e.g., +1, -2, opposite letters).
• Number/Symbol Coding: Letters are assigned specific numerical values or symbols.
• Substitution Coding: Names of objects are substituted with other names (e.g., if ‘white’ is called ‘blue’, ‘blue’ is called ‘red’).
• Deciphering Message Word Codes: A few sentences are given in a coded language, and you have to find the code for a specific word by finding common words across sentences.

3. Solved Examples with Step-by-Step Explanations

Example 1: Letter Coding (Shift Pattern)

Question: In a certain code, TEACHER is written as VGCEJGT. How is CHILDREN written in that code?

Step-by-step Explanation:
1. Analyze the given code: T -> V, E -> G, A -> C, C -> E, H -> J, E -> G, R -> T.
2. Find the numerical shift: T(20) to V(22) is +2. E(5) to G(7) is +2.
3. The pattern is clearly +2 for all letters.
4. Apply this to CHILDREN:
C(+2) = E
H(+2) = J
I(+2) = K
L(+2) = N
D(+2) = F
R(+2) = T
E(+2) = G
N(+2) = P
Answer: EJKNFTGP

Example 2: Number Coding

Question: If CAT is coded as 24 and SAD is coded as 24, how is SHE coded?

Step-by-step Explanation:
1. Write down positional values for CAT: C=3, A=1, T=20.
2. Add them up: 3 + 1 + 20 = 24. This matches the code.
3. Verify with SAD: S=19, A=1, D=4. Sum = 19 + 1 + 4 = 24. (Verified).
4. Apply to SHE: S=19, H=8, E=5.
5. Sum = 19 + 8 + 5 = 32.
Answer: 32

Example 3: Substitution Coding

Question: If ‘orange’ is called ‘butter’, ‘butter’ is called ‘soap’, ‘soap’ is called ‘ink’, ‘ink’ is called ‘honey’ and ‘honey’ is called ‘orange’, which of the following is used for washing clothes?

Step-by-step Explanation:
1. First, find the factual answer to the question: What is used for washing clothes? The answer is ‘soap’.
2. Now, look at the given substitutions. What is ‘soap’ called in this code?
3. The problem states: ‘soap’ is called ‘ink’.
4. Therefore, the code for the washing item is ‘ink’. (Do not go further and check what ‘ink’ is called).
Answer: ink

Example 4: Message Decoding (Chinese Coding)

Question: In a certain code language,
‘pit na som’ means ‘bring me water’
‘na jo tod’ means ‘water is life’
‘tub od pit’ means ‘give me toy’
What is the code for ‘bring’?

Step-by-step Explanation:
1. Compare statement 1 and 2: ‘pit na som’ and ‘na jo tod’. The common word in English is ‘water’. The common code word is ‘na’. So, ‘na’ = ‘water’.
2. Compare statement 1 and 3: ‘pit na som’ and ‘tub od pit’. The common word in English is ‘me’. The common code word is ‘pit’. So, ‘pit’ = ‘me’.
3. Look at statement 1 again: ‘pit na som’ means ‘bring me water’. We know ‘pit’ = ‘me’ and ‘na’ = ‘water’.
4. Therefore, the remaining code ‘som’ must mean the remaining word ‘bring’.
Answer: som

4. Pro-Tips to Avoid Common Mistakes

• Don’t Assume the Pattern Holds: In letter coding, always check the pattern for at least 3-4 letters of the given word. Sometimes, the pattern changes halfway (e.g., first half +1, second half -1).
• Beware of Vowels vs. Consonants: Some difficult questions have different coding rules for vowels and consonants. If uniform addition/subtraction fails, check if vowels are behaving differently.
• Read Substitution Questions Carefully: In substitution coding (Example 3), stop exactly at what the item is called. Don’t create a chain (e.g., soap -> ink -> honey).
• Use Elimination for Message Coding: In message deciphering, quickly strike out the common words in both sides using a pencil. This avoids confusion and saves time.
• Reverse Coding: Sometimes the word is reversed, and then the shift (+1, +2) is applied. If direct mapping makes no sense, try reversing the word first.

5. Practice Questions

1. If in a certain language, MADRAS is coded as NBESBT, how is BOMBAY coded in that language?
2. If GO=32, SHE=49, then SOME will be equal to?
3. If ‘sky’ is ‘star’, ‘star’ is ‘cloud’, ‘cloud’ is ‘earth’, ‘earth’ is ‘tree’ and ‘tree’ is ‘book’, then where do the birds fly?
4. In a certain code, TRIPPLE is written as SQHOOKD. How is DISPOSE written in that code?
5. In a certain code language, ‘743’ means ‘mangoes are good’, ‘657’ means ‘eat good food’ and ‘934’ means ‘mangoes are ripe’. Which digit means ‘ripe’ in that language?

Answers for Practice Questions

1. CPNCBZ (Pattern: +1 to each letter)
2. 56 (Pattern: Sum of opposite letter positions. G’s opposite is T(20), O’s opposite is L(12). 20+12=32. SOME = H(8)+L(12)+N(14)+V(22) = 56)
3. star (Birds fly in the ‘sky’. And ‘sky’ is called ‘star’)
4. CHRONRD (Pattern: -1 for each letter)
5. 9 (From statement 1 and 3, common words are ‘mangoes are’, and common digits are 4, 3. In ‘934’, remaining word is ‘ripe’, remaining digit is 9. So ripe = 9).