Letter Coding: Shifts, Reversals and Letter-to-Number Codes
Letter Coding: Shifts, Reversals and Letter-to-Number Codes
In coding-decoding, a word is turned into a code by a fixed rule, and you apply the same rule to a new word (or undo it). Write the positions of the letters under the word and the code; the rule shows up as a pattern of differences. NTPC sets uniform shifts, position-wise shifts, reversals and opposite letters, and letters turned into numbers.
1. Rule Finder
| Look at | Rule it reveals | Example |
|---|---|---|
| Same shift for every letter | Uniform shift ±n | CAT → ECV (+2) |
| Shift grows with position | +1, +2, +3 ... or −1, −2, −3 ... | BOOK → CQRO |
| Code letters sum to 27 with the word's letters | Opposite letters | CAT → XZG |
| Code reads backwards | Word reversed, sometimes then shifted | LAMP → PMAL → QNBM (+1) |
| Code is digits | Positions, reverse positions (A = 26) or position ± k joined | DOG → 4 15 7 or 23 12 20 |
Positions: EJOTY = 5, 10, 15, 20, 25. Opposite letter = 27 − position
Opposite pairs worth memorising: A-Z, B-Y, C-X, D-W, E-V, F-U, G-T, H-S, I-R, J-Q, K-P, L-O, M-N
To decode, apply the rule backwards: a +2 code is decoded by −2
✗ MOBILE → LNAHKD is a −1 code, so decode QZHKVZX by another −1 | ✓ Decoding reverses the rule: +1 gives RAILWAY
✗ The shift past Z is impossible | ✓ Wrap round: Y + 3 = B, A − 3 = X
हिंदी नोट: कूट-लेखन में शब्द और कोड के अक्षरों की स्थिति नीचे लिखिए; अंतर से नियम दिखेगा। विपरीत अक्षरों की स्थिति का योग 27 होता है। डिकोड करने के लिए नियम उल्टा लगाइए।
Exam Pointer: Verified NTPC patterns: uniform letter shifts (January 2021 CBT-1), position-wise shifts (June 2022 CBT-2), and letters written as reverse place values or position values joined together (two January 2021 CBT-1 shifts). Reversed-word and opposite-letter codes had no separate verified shift and are tagged syllabus-based.
Pariksha Pattern: Every Way NTPC Asks This Topic
Pattern 1: Uniform shift
[PYQ: NTPC CBT-1 12-Jan-2021 Shift-1]
EXAM LEVEL
Q. If CAT is coded ECV, how is DOG coded?
Each letter moves +2 (C → E, A → C, T → V). DOG becomes FQI.
Answer: FQI
EXAMATLAS LEVEL
Q. If MOBILE is written LNAHKD, which word is written QZHKVZX?
Each letter moved −1 to make the code, so move each code letter +1: Q → R, Z → A, H → I, K → L, V → W, Z → A, X → Y.
Answer: RAILWAY
Pattern 2: Position-wise shift
[PYQ: NTPC CBT-2 15-Jun-2022 Shift-3]
EXAM LEVEL
Q. The 1st letter moves +1, the 2nd +2, the 3rd +3 and the 4th +4. Code BOOK.
B → C, O → Q, O → R, K → O.
Answer: CQRO
EXAMATLAS LEVEL
Q. TRAIN is written SPXEI. How is METRO written?
T → S (−1), R → P (−2), A → X (−3, wrapping), I → E (−4), N → I (−5). For METRO: M − 1 = L, E − 2 = C, T − 3 = Q, R − 4 = N, O − 5 = J.
Answer: LCQNJ
Pattern 3: Opposite letters and reversed words
[Pattern: syllabus-based, PYQ-style]
EXAM LEVEL
Q. Each letter is replaced by its opposite (A ↔ Z). Code CAT.
C (3) ↔ X (24), A ↔ Z, T (20) ↔ G (7).
Answer: XZG
EXAMATLAS LEVEL
Q. LAMP is coded QNBM. How is DESK coded?
LAMP reversed is PMAL, and each letter +1 gives QNBM. DESK reversed is KSED, and +1 gives LTFE.
Answer: LTFE
Pattern 4: Letters to numbers
[PYQ: NTPC CBT-1 16-Jan-2021 Shift-2 | NTPC CBT-1 19-Jan-2021 Shift-2]
EXAM LEVEL
Q. If A = 26, B = 25 and so on, write DOG as these numbers joined together.
Reverse values: D = 23, O = 12, G = 20.
Answer: 231220
EXAMATLAS LEVEL
Q. In a code, each letter is written as its position plus 2 and the numbers are joined, so BAT = 4322. How is LAMP written?
L = 12 + 2 = 14, A = 1 + 2 = 3, M = 13 + 2 = 15, P = 16 + 2 = 18.
Answer: 1431518
60-Second Revision
- Write positions under the word and code; read the differences.
- Uniform, position-wise, reversed or opposite: test in that order.
- Decode by running the rule backwards.
- Number codes: position, reverse position (27 − n) or position ± k.