MPESB Group · Bite-size lessons

Coding-Decoding

About 6 min · Reasoning and Aptitude · English

Time worked out from the lesson's 410 words at 150 words a minute, plus a minute for each question.

Coding-Decoding is a fundamental topic in the Reasoning and Aptitude section of MPESB Group 1/2/3 exams. It tests your ability to identify hidden patterns and apply logical rules consistently. In these questions, words, letters, or numbers are converted into codes using a specific system, and you must either decode a given code or encode a new word using the same logic.

Key ideas

• Letter Position System: Every letter has a fixed position — A=1, B=2, C=3... Z=26. Most coding patterns involve adding, subtracting, or manipulating these positions.
• Forward and Backward Shifting: Letters may be shifted forward (+) or backward (−) by a fixed number. Example: If each letter moves +2, then A→C, B→D, CAT→ECV.
• Opposite Letter Pairing: Each letter pairs with its "opposite" such that their positions sum to 27. A↔Z, B↔Y, C↔X, D↔W... M↔N.
• Reversal Coding: The word or its code is simply reversed. BOOK coded as KOOB, or the coded letters themselves are reversed after applying another rule.
• Position-Based Variable Shifting: Different letters shift by different amounts based on their position in the word. First letter +1, second letter +2, third letter +3, and so on.

Watch out for

• Forgetting to wrap around at Z: When shifting A by −2, students write "negative" instead of Y. Fix: If result is 0 or negative, add 26. If result exceeds 26, subtract 26.
• Confusing opposite coding with reversal: Opposite coding uses the formula 27−position (A↔Z), while reversal just flips the word order (CAT→TAC). Always test both if unsure.
• Assuming uniform shift without checking all letters: The first letter might shift by +2, but other letters might follow a different pattern (variable shift). Fix: Always verify the pattern for at least 2–3 letters before applying.

Quick reference

• Opposite letter: Position + Opposite position = 27 (A↔Z, B↔Y, M↔N)
• Shift coding: Add/subtract a constant, wrap around at 26
• Variable shift: First letter +1, second +2, third +3... check position-wise
• Number codes: Usually sum of positions or concatenated positions
• Substitution coding: Find the common word across two statements to isolate codes

Check yourself: 3 questions

This lesson is cut from the topic's note, not rewritten. Fact-checked on 17 Oct 2026: an AI-assisted check of this note against cited sources; the errors it found were corrected that day. Some errors may remain. The questions are from Shishya's validated question bank.

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