Coding Decoding Questions and Answers: A Guide for UPSC CSAT

Arvin_G

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Mar, 2025

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4 min read

In the UPSC CSAT, mastering logical reasoning is essential. One of the most crucial components of logical reasoning is coding-decoding questions. But what exactly are coding-decoding questions, and why are they so important for aspirants? Simply put, coding-decoding involves identifying patterns, sequences, and relationships to solve logical puzzles.

For the UPSC Prelims CSAT, coding-decoding questions test your ability to think critically under time constraints. From rearranging letters to deciphering numerical codes, these problems challenge your mental agility, making them a key area of preparation.

Ready to ace this section and improve your CSAT score? Let’s get started!

Understanding the UPSC Prelims CSAT Paper

The CSAT, which is part of the UPSC Prelims, is a qualifying paper. To pass this paper, candidates must secure a minimum of 33% marks. This means you need to score at least 66 out of 200 marks to qualify for the General Studies Paper I. The marks obtained in CSAT are not counted for the final merit list but are crucial for determining eligibility for the next stage of the UPSC examination.

The CSAT paper tests your skills in Comprehension, Logical Reasoning, Analytical Ability, and Basic Numeracy.

The paper consists of 80 questions.
All questions are of multiple-choice type (MCQs) with four different options.
The exam duration is 2 hours.
Each question carries 2.5 marks.
There is a negative marking of 1/3rd (0.83) of the marks for each incorrect answer.

Also read: Useful Websites For UPSC And IAS Preparation

Understanding What is Coding-Decoding

Coding-decoding is a logical reasoning concept where information is presented in a specific code, and you must interpret or transform it based on given rules. In simple terms, it involves identifying patterns applied to letters, numbers, or symbols to determine the original or intended meaning.

Major Types of Coding-Decoding Questions

Here are the major types of coding-decoding questions commonly encountered in the UPSC CSAT, along with examples.

1. Letter Coding

In this type, letters are transformed based on specific rules such as shifting positions in the alphabet or replacing them with other letters.

Example: In a certain code, "DOG" is written as "FQI." How is "CAT" written in that code?

Solution: Each letter is shifted two steps forward in the alphabet. D → F, O → Q, G → I
C → E, A → C, T → V

Answer: "CAT" is written as "ECV."

2. Number Coding

Here, letters or words are assigned numerical values following a specific pattern or logic.

Example: In a certain code, each letter of the alphabet is assigned a value equal to its position in the alphabet (i.e., A = 1, B = 2, ..., Z = 26). What is the code for the word "ACE"?

Solution: Assign values to each letter using their position in the alphabet: A = 1, C = 3, E = 5. Add the values: 1 + 3 + 5 = 9

Answer: The code for "ACE" is 9.

3. Substitution Coding

Words are replaced with other words, symbols, or phrases according to a substitution rule.

Example: In a certain code language, "Apple" is called "Mango," "Mango" is called "Banana," "Banana" is called "Grapes," and "Grapes" is called "Apple." What is yellow in color in this code language?

Solution: In reality, a Banana is yellow. According to the code, "Banana" is called "Grapes."

Answer: The object that is yellow is "Grapes."

4. Mixed Letter and Number Coding

This involves a combination of letters and numbers in a coded format.

Example: If "X1Y2Z3" is written as "Z3X1Y2," how is "M4N5O6" written?

Solution: The last part moves to the front, followed by the remaining sequence. "M4N5O6" → "O6M4N5"

Answer: O6M4N5

5. Deciphering Coding

Scrambled words or phrases are decoded into meaningful ones using logical analysis.

Example: If "TAC" is the code for "CAT," what is the code for "BIRD"?

Solution: Reverse the order of letters. "BIRD" → "DRIB"

Answer: The code for "BIRD" is "DRIB."

Now that you’ve grasped the core concepts, it’s time to practice! Keep solving and refining your approach to ace this section with confidence.

Coding-Decoding Practice Questions with Solutions

Sample Question 1: In a certain code language, MANGO is written as NCOMH, and APPLE is written as BQOKD. How is GRAPE written in that code?

Solution: Observing the pattern:

Each letter is shifted alternately:
- M → N (+1), A → C (+2), N → O (+1), G → M (+2), O → H (-1)
- A → B (+1), P → Q (+1), P → O (-1), L → K (-1), E → D (-1)

Following the same pattern for GRAPE:

G → H (+1), R → T (+2), A → B (+1), P → N (-1), E → D (-1)
Answer: HTBND

Sample Question 2: In a certain code, numbers are assigned to letters as follows:
A = 2, B = 4, C = 6, D = 8, ..., Y = 50, Z = 52. What is the sum of the coded values for the word UPSC?

Solution: From the given pattern, each letter is assigned a value as position × 2:

U = 42 (21 × 2)
P = 32 (16 × 2)
S = 38 (19 × 2)
C = 6 (3 × 2)

Sum = 42 + 32 + 38 + 6 = 118

Answer: 118

Practice Questions for Learning

1. In a certain code, DELHI is written as EDJKM, and MUMBAI is written as NVMCFK. How is KOLKATA written in this code?

(a) LPMMBXB

(b) LPNNCXB

(d) LPMNCXA

2. In a certain coding language, "AIM HIGH" is coded as ZRL SGSR. What is the code for "WORK HARD"?

(a) XLPK IBSE

(b) VNQJ GCQC

(d) XNQK IBSE

3. If in a certain code, TIGER is written as VKEGT, how is LION written in that code?

(a) NJPM

(b) NJPO

(d) NKPO

4. If APPLE = 50 and ORANGE = 74, what is the code for BANANA?

(a) 56

(b) 52

(d) 54

5. In a certain code language, DOOR = 118 and WINDOW = 140. What is the code for HOUSE?

(a) 122

(b) 130

(d) 128

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Also read: Top CSAT Books for UPSC Prelims Exam

UPSC CSAT Previous Year’s Coding-Decoding Questions with Solutions

Understanding and practicing the previous year's questions is one of the most effective ways to prepare for coding and decoding questions in the UPSC CSAT exam. In this section, we’ll go over some of the most frequently asked coding-decoding questions from previous years, along with step-by-step solutions, to help you sharpen your problem-solving skills for the exam.

Question: This is a PYQ from UPSC CSE Prelims 2024 CSAT General Studies Paper II:

Explanation: To solve the question, we analyze the given examples to identify the coding pattern:

1. POT → ATOP

Step 1: Reverse the original word: TOP
Step 2: Add the letter "A" at the beginning: A + TOP
Encoded Word: ATOP

2. TRAP → APART

Step 1: Reverse the original word: PART
Step 2: Add the letter "A" at the beginning: A + PART
Encoded Word: APART

3. Applying the Pattern to ‘ARENA’

Step 1: Reverse the word: ANERA
Step 2: Add the letter "A" at the beginning: A + ANERA

Answer: d) AANERA

Must read: CSAT 2024 in UPSC: Exam Pattern, Qualifying Marks, and Syllabus

Question: This is a PYQ from UPSC CSE Prelims 2023 CSAT General Studies Paper II:

Explanation: The given word ‘ZERO’ is encoded as ‘CHUR’. Let’s decode the pattern step by step:

Step 1: Identify the Rule

Each letter in the word ‘ZERO’ is replaced by the letter that is three places ahead of it in the English alphabet.

Z → C: Z is the 26th letter. Adding 3 gives 29. Since the alphabet has only 26 letters, subtract 26, resulting in the 3rd letter: C.
E → H: E is the 5th letter. Adding 3 gives 8, which corresponds to H.
R → U: R is the 18th letter. Adding 3 gives 21, which corresponds to U.
O → R: O is the 15th letter. Adding 3 gives 18, which corresponds to R.

Thus, ‘ZERO’ → ‘CHUR’.

Step 2: Apply the Rule to ‘PLAYER’

P → S: P is the 16th letter. Adding 3 gives 19, which corresponds to S.
L → O: L is the 12th letter. Adding 3 gives 15, which corresponds to O.
A → D: A is the 1st letter. Adding 3 gives 4, which corresponds to D.
Y → B: Y is the 25th letter. Adding 3 gives 28. Subtract 26 to get 2, which corresponds to B.
E → H: E is the 5th letter. Adding 3 gives 8, which corresponds to H.
R → U: R is the 18th letter. Adding 3 gives 21, which corresponds to U.

Answer: (d) ‘PLAYER’ → ‘SODBHU’

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Question: This is a PYQ from UPSC CSE Prelims 2022 CSAT General Studies Paper II:

Explanation: To determine how ‘LUCKNOW’ is encoded, we reverse the order of the English alphabet and assign positions accordingly. In the reversed alphabet:

A (1st letter) becomes Z (26th letter),
B (2nd letter) becomes Y (25th letter),
and so on.

The reverse position of a letter can be calculated as:
Reverse Position = 27 - Original Position

Step-by-Step Decoding:

L: Original Position = 12

Reverse Position = 27 - 12 = 15
Letter at 15th position = O

U: Original Position = 21

Reverse Position = 27 - 21 = 6
Letter at 6th position = F

C: Original Position = 3

Reverse Position = 27 - 3 = 24
Letter at 24th position = X

K: Original Position = 11

Reverse Position = 27 - 11 = 16
Letter at 16th position = P

N: Original Position = 14

Reverse Position = 27 - 14 = 13
Letter at 13th position = M

O: Original Position = 15

Reverse Position = 27 - 15 = 12
Letter at 12th position = L

W: Original Position = 23

Reverse Position = 27 - 23 = 4
Letter at 4th position = D

Final Encoding: ‘LUCKNOW’ → ‘OFXPMLD’

Answer: (d) OFXPMLD

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Common Mistakes to Avoid While Solving Coding-Decoding Questions

While solving coding-decoding questions, aspirants often make errors that can be avoided with proper strategy. Here are some common mistakes and how to tackle them:

1. Misinterpretation of Codes

One of the most common mistakes is misunderstanding the pattern of coding.

Example: If "SUN" is coded as "TVP," how is "MOON" coded?

Incorrect approach: Assuming each letter is replaced with its next letter.

Correct approach: Identifying the actual pattern used in the given code. Instead of blindly following assumptions, break down the logic step by step.

2. Making Assumptions

Many aspirants assume a pattern without verifying it across all letters or numbers.

Example: If "RAT" is coded as "SBU," and "DOG" as "EPH," what is the code for "BAT"?

Incorrect approach: Guessing without checking for consistency.

Correct approach: Identifying that each letter moves one step forward in the alphabet. Always check whether a rule applies consistently before applying it.

3. Spending Excessive Time on One Question

Some candidates waste too much time on a single question, affecting overall performance.

Tip: If you find a question difficult, mark it for review and move forward. Time management is key.

4. Lack of Regular Practice

Since coding-decoding questions require logical pattern recognition, a lack of practice can lead to slower solving speed and errors.

Solution: Solve a variety of practice questions daily to improve pattern recognition skills.

Also watch: UPSC Prelims 2024: Master the last 40 days with this SMART Strategy!

Final Thoughts

Coding-decoding questions in the UPSC CSAT are an essential part of logical reasoning and can be highly scoring with the right approach. Mastering different types of coding-decoding questions will enhance your problem-solving speed and accuracy.

Stay consistent, keep practicing, and make coding-decoding your strong suit to enhance your performance in logical reasoning sections!