Coding-Decoding — Study Notes

Overview

Coding-Decoding is a fundamental reasoning topic in SSC MTS where information (letters, numbers, or words) is encrypted using a specific rule, and you must either decode the given code or encode new information using the identified pattern. This topic tests your pattern recognition ability and logical thinking under time pressure.

In SSC MTS Paper 1, expect 2–4 questions from this area. The codes are generally straightforward—letter shifts, number substitutions, or conditional rules based on word properties. Mastering the common patterns allows you to solve most questions in under 30 seconds each. The key skill is quickly spotting whether the code is based on position values, forward/backward shifts, or conditional logic.

Success here requires systematic checking: always compare multiple examples to confirm your hypothesis about the coding rule. Don't assume a pattern from just one word—verify with all given pairs before applying it to the answer.

Key Concepts

• **Letter Coding**: Each letter is replaced according to a fixed rule—most commonly alphabetical position shifts (forward or backward), reverse alphabet substitution, or position-based arithmetic operations.

• **Number Coding**: Words or letters are converted to numbers based on alphabetical positions (A=1, B=2... Z=26), then arithmetic operations (addition, multiplication, difference) are applied.

• **Substitution Coding**: Direct replacement where specific letters/numbers are swapped with predetermined symbols or other letters, creating a cipher-like code.

• **Conditional Coding**: The coding rule changes based on word characteristics like number of letters, vowel/consonant pattern, or position of specific letters within the word.

• **Pattern Verification**: Always test your identified rule against ALL given examples before selecting an answer—one match is coincidence, consistent matches confirm the pattern.

• **Reverse Engineering**: In decoding questions, work backward from the code to the original; in encoding questions, apply the discovered rule forward to new inputs.

Formulas / Key Facts

1. **Alphabetical Positions**: A=1, B=2, C=3... Z=26 (memorize positions of at least A–J for quick calculation)

2. **Reverse Alphabet**: A↔Z, B↔Y, C↔X... Formula: Reverse of nth letter = (27-n)th letter

3. **Position Shift**: If code shifts by +k, new position = (original position + k); handle wrap-around using modulo 26

4. **Common Shift Values**: +1, +2, +3, -1, -2 (majority of SSC MTS questions use these simple shifts)

5. **Vowels in English**: A, E, I, O, U (positions 1, 5, 9, 15, 21—useful for conditional coding)