Coding-Decoding Questions for SSC CHSL

Letter Coding (Shifting Pattern)

In this type, each letter in a word is replaced by another letter that is a fixed number of steps forward or backward in the alphabet. For example, if A becomes D, then every letter moves 3 steps forward. Always check the first two letters of the word and find the gap. That same gap will apply to all letters. If the shift is forward, add the number. If backward, subtract. Remember the alphabet ends at Z (26) and wraps around, so Z plus 1 becomes A again.

• Find shift by comparing original letter position with coded letter position.
• Use A=1, B=2 ... Z=26 to calculate positions quickly.
• If shift is the same for all letters, it is a simple Caesar cipher.
• Sometimes even positions shift differently from odd positions — check alternate letters separately.
• Reverse coding: if COME is coded as XLNV, each letter is replaced by its mirror (A-Z, B-Y, C-X) — this is called opposite coding.
• Mnemonic: ATBASH for opposite coding — A pairs with Z, T pairs with G.

Number Coding

Here, letters are replaced by numbers using a specific rule — could be their position values, reversed position values, or some arithmetic applied on them. For example, A=1, B=2 or A=26, B=25 (reverse). Sometimes each letter is assigned a two-digit code and you read them together as a number. Always decode at least two words before answering. One example is never enough to confirm the rule in number coding.

• Standard: A=1 to Z=26. Reverse: A=26 to Z=1.
• Sometimes the code number = position + constant (like position + 2).
• Look for whether vowels and consonants are coded differently.
• Sum coding: the code of the whole word = sum of letter positions.
• Product or multiplication coding is rare but has appeared in recent papers.
• Always verify your found rule with a second word given in the question.

Word Coding (Substitution)

In this type, actual words are substituted for other words. For example, sky is called green, green is called blue, blue is called red. Then the question asks: what is the color of grass? Grass is green, but green is called blue — so the answer is blue. This is a trick type. Do not answer with the logical real-world answer. Always replace the final answer word with what it is called in the coded language.

• Read the chain of substitutions carefully.
• First find the real-world correct answer, then find what that word is called.
• Do not replace twice — only one substitution at the end.
• These questions take 30 to 45 seconds if you stay calm and follow the chain.
• Mnemonic: Real answer first, then swap once — RAFTS (Real Answer, Find Then Swap).

Mixed or Conditional Coding

Here, three or four sentences are coded and you must find the code for a single word by comparing sentences. If two sentences share one common word and their codes share one common code, that code belongs to that word. This is like solving simultaneous equations. Eliminate the extras to isolate the target word.

• Compare two sentences that differ by only one word — the differing code = the differing word.
• Use process of elimination across all given coded sentences.
• Write out which words are common between sentences and match their codes.
• Order of code words usually does not match order of original words — do not assume left-to-right.
• These questions take 60 to 90 seconds — attempt them after easier ones in the set.

• Assuming the coding is always A=1 without checking — always verify with at least two letters before committing to a rule.
• In word substitution, giving the real-world answer instead of the coded answer — for example, saying green instead of blue for grass color.
• In letter shifting, forgetting to wrap around at Z. After Z comes A, not ZA or AA.
• In mixed coding, assuming the first word of the sentence matches the first code — positions can be jumbled.
• Rushing through with one example verified — always cross-check with a second word or letter to confirm the pattern.

• ATBASH: A pairs with Z, B with Y — remember it as the alphabet folded in half and mirrored.
• RAFTS for word coding: Real Answer First, Then Swap — stops you from giving the uncoded answer.
• For shifting, think of a clock: go past 12 (Z) and you restart at 1 (A) — circular shift.
• In mixed coding, think of it like a Venn diagram: the word in the overlap of two sentences = the code in the overlap of their codes.

• SSC CHSL typically gives 3 to 4 coding-decoding questions per paper, mostly letter shifting and mixed coding. Word substitution appears once in a while.
• Letter shifting questions are the fastest to solve — under 30 seconds if you know A=1 to Z=26 by heart. Memorize this before exam.
• Mixed coding questions are slightly harder and take more time. In the exam, if stuck, skip and return — do not let one question eat 3 minutes.
• Recent CHSL papers (2023-2024) have shown more mixed coding and number coding than pure letter shifting, so practice those types more.
• Never spend more than 60 seconds on any single coding-decoding question. These are scoring questions but not worth losing time for other sections.