Coding-Decoding Questions for RRB NTPC

Type 1: Letter Shifting (Forward and Backward)

In this type, each letter of the original word is shifted forward or backward by a fixed number of positions in the alphabet. For example, if A becomes D, B becomes E — each letter has moved +3. Always check the shift for the first 2-3 letters to confirm the pattern is consistent before applying it to the unknown word. Shifting can also be alternate: odd-position letters shift +2, even-position letters shift -1.

• Always write A=1, B=2 ... Z=26 on your rough sheet at the start.
• Check shift for at least 2 letters before concluding.
• If shift crosses Z, wrap around: Z+1 = A (circular shift).
• Backward shift: if A becomes X, shift is -3 (or +23 circular).
• Alternate shifts are common: +2 for odd positions, -2 for even positions.
• Reverse order coding: CAT coded as TAC — just reverse the letters.

Type 2: Number-Based Coding

Each letter is replaced by its position number in the alphabet, sometimes modified by adding, subtracting, or multiplying. Another common version assigns numbers based on the word's letter count or rank. In some questions, words are coded as numbers that represent the sum or product of letter positions.

• A=1, B=2, C=3 ... Z=26. Memorize this instantly.
• Some codes use reverse positions: A=26, B=25, Z=1.
• Sum-based: BALL = 2+1+12+12 = 27 coded as 27.
• Product or difference patterns also appear occasionally.
• Always cross-check the pattern with both the given examples before answering.

Type 3: Symbol and Mixed Substitution

Here, letters or words are replaced by symbols, numbers, or other letters based on a defined table or pattern given in the question itself. These questions always give you the code key — you just need to apply it carefully. The trick is to read slowly and not mix up two similar-looking symbols.

• The code key is always given in the question — use it, do not guess.
• Write the mapping out on rough paper before decoding.
• Watch out for similar symbols: * and + or @ and #.
• Some questions code entire words (not letters): TABLE means CHAIR type.
• In word-substitution, eliminate answer choices using known substitutions.

Type 4: Sentence or Word-Level Coding

A full sentence is coded and you must find the code for a single word. The exam gives you 2-3 coded sentences with common words to help you decode. Find the word that appears in two sentences — its code appears in both coded sentences, letting you isolate it. This is a logic puzzle more than a math one.

• Look for the common word between two sentences — its code is the common code.
• Use elimination: if one sentence has 3 words and one is known, decode the rest.
• Word order may or may not be preserved — check both possibilities.
• Never assume word order is same as coded order unless confirmed.
• These questions take slightly more time — attempt after easier ones.

Type 5: Reverse and Positional Tricks

Some codes are formed by reversing the word, interchanging adjacent letters, or using only alternate letters. DELHI coded as IHLED is simply reversed. CLOUD coded as LDOCU means pairs of letters are swapped: CL swap to LC, OU swap to UO, D stays. Recognizing these tricks quickly is the key skill.

• Reverse coding: write the word backwards.
• Pair swapping: swap letters in pairs (1-2, 3-4, 5-6...).
• Odd-even position swap: 1st and 2nd letter swap, 3rd and 4th swap.
• First and last letter interchange is another common pattern.
• Always check with at least one full example before locking the pattern.

• Confusing +3 shift with -3 shift: always verify direction by testing the first letter of the given coded example.
• Forgetting circular wrap-around: X+3 is not X3 but A (since X=24, 24+3=27, 27 mod 26 = 1 = A).
• In sentence coding, assuming word order is preserved when it may not be — always cross-check with two sentences.
• Mixing up normal alphabet values (A=1) with reverse alphabet values (A=26) — the question type looks similar but the logic is opposite.
• Rushing through symbol-substitution and misreading a symbol — slow down for 10 seconds, it saves 30 seconds of re-doing.

• SHIFT CHECK: Always test the pattern on 2 letters before applying. Think S-C: Sample before Coding.
• Alphabet row trick: Write ABCDEFGHIJKLMNOPQRSTUVWXYZ and number them 1-26 on your rough sheet in the first 30 seconds of the exam. Saves time every question.
• Reverse value shortcut: Reverse value of any letter = 27 minus its normal value. M=13, reverse = 27-13 = 14 = N. Easy!
• For sentence coding, think of it like a Venn diagram: the word appearing in two sentences is your decoder key — the common element unlocks everything.

• RRB NTPC typically asks 3-5 coding-decoding questions per paper. In CBT-1, they are usually easy to medium. In CBT-2, expect at least one sentence-level coding question which is slightly tricky.
• Letter shifting (+n or -n) is the single most asked pattern. If you master only one type, master this. It appears in nearly every paper.
• Difficulty is low-medium — these should be your confidence questions. Target 100% accuracy here, not speed shortcuts that cause errors.
• Recent papers (2019-2021 cycles) showed a rise in mixed-type coding where two patterns are combined (e.g., shift +2 AND reverse). Expect one such question.
• Time target: easy coding = 30-45 seconds, sentence-level coding = 60-90 seconds. If a question crosses 90 seconds, mark and move on — come back later.