TNPSC Group IV · Bite-size lessons
Alphanumeric Reasoning
About 6 min · Aptitude and Mental Ability Test · English
Time worked out from the lesson's 407 words at 150 words a minute, plus a minute for each question.
Alphanumeric reasoning tests your ability to identify patterns in letters, numbers, and their combinations. The two main areas tested are coding-decoding (where words are converted using a rule) and alphabet tests (finding letter positions, sequences, and patterns).
Key ideas
- Alphabet Position Values: Every letter has a fixed position—A=1, B=2... Z=26. This is the foundation for all alphanumeric problems. Also memorize reverse positions: Z=1, Y=2... A=26.
- Coding-Decoding Principle: A word is transformed into a code using a consistent rule. Your job is to either find the rule from examples or apply a given rule to a new word.
- Letter Shifting: The most common coding pattern. Each letter moves forward or backward by a fixed number. Example: +2 shift means A→C, B→D, C→E.
- Position-Based Coding: Letters are replaced by their position numbers or by calculations involving positions. Example: CAT → 3-1-20 or 3+1+20=24.
- Opposite Letter Pairing: A pairs with Z, B with Y, C with X, and so on. The sum of positions of opposite letters is always 27.
Watch out for
- Confusing left and right directions: "5th to the left of K" means K minus 5 positions, not plus. Draw the alphabet line if unsure. → Always visualize: left means subtract, right means add.
- Forgetting cyclic continuation: When shifting W by +5, students write "beyond Z" instead of continuing from A. W+5 = X, Y, Z, A, B. → After Z comes A; after A going backward comes Z.
- Miscounting positions under pressure: Counting H as 7 instead of 8 because students start from zero or skip a letter. → Use EJOTY checkpoints: H is 3 after E(5), so H=8.
Quick reference
- EJOTY: E=5, J=10, O=15, T=20, Y=25—use as checkpoints for quick position finding.
- Opposite pairs sum to 27: A-Z, B-Y, C-X... useful for reverse alphabet coding.
- Cyclic alphabet: After Z comes A; position 27 = position 1 = A.
- Left = subtract, Right = add when counting from a given letter.
- Check all examples: Never assume a pattern from just one letter pair—verify with all given data.
Check yourself: 3 questions
1. In a certain code language, 'BOOK' is written as 'CPPL'. How will 'DESK' be written in that code?
2. If in the English alphabet, every third letter is dropped starting from A (i.e., A, D, G... are dropped), what will be the 10th letter in the remaining sequence?
3. In a certain code, 'MASTER' is written as '13-1-19-20-5-18'. On the same basis, how will 'PENCIL' be written?
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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