Coding–decoding for SSC CHSL

A coding question shows you a word and its code and asks you to code another word the same way. The rule is hidden, but not deeply. SSC CHSL draws on perhaps seven or eight rule types, and most questions use one of the first four.

The method never changes: write the positions, compare letter by letter, name the rule, apply it. Candidates who try to "see" the rule without writing positions are fast on easy questions and lost on anything with alternating shifts.

How it is asked

• Letter to letter: "If BOOK is written as CPPL, how is DESK written?"
• Letter to number: "If CAT = 24 and DOG = 26, what is BEE?"
• Letter to digit: each letter stands for a fixed digit, and you build a new word's code from known letters.
• Sentence codes: several sentences with their codes; find the code for one word.
• Substitution: "If table is called chair and chair is called bed…"
• Decoding: the reverse: given the code, find the word.

The method, step by step

1. Write positions under each letter of the word and its code (A1 … Z26; EJOTY helps).
2. Find the change for each letter: +1, −2, and so on.
3. Name the rule.
   • All changes equal → uniform shift.
   • Changes alternate (+1, −1, +1…) → alternating shift.
   • No pattern → compare the code with the word reversed, then with the opposite letters.
4. Check the rule on every letter of the example, not just the first two.
5. Apply it to the new word, and match the result against the options letter by letter.

For number codes, try the sum of positions first, then reverse positions (27 − n), then products or the number of letters.

The rule types

Why reversal is checked third: a reversed word often looks like a random shift. If the differences between word and code look chaotic, compare the code with the word written backwards before you try anything harder.

Six worked questions

Q1. If TRAIN is written as UQBHO, how is PLANE written? (a) QKZMF (b) QMBOF (c) OKBMD (d) QKBMF
T→U is +1, R→Q is −1, A→B is +1, I→H is −1, N→O is +1. Apply +1, −1, +1, −1, +1 to PLANE: P→Q, L→K, A→B, N→M, E→F. Answer: (d) QKBMF.

Q2. If COLD is written as EMPD, how is WARM written? (a) XBSN (b) NSBX (c) NSBY (d) MRAW
The shifts are C→E +2, O→M −2, L→P +4, D→D 0, which is no pattern. Reverse COLD to get DLOC, and compare: D→E, L→M, O→P, C→D, each +1. So the rule is "reverse, then +1". WARM reversed is MRAW, and +1 gives NSBX. Answer: (b) NSBX.

Q3. If ACT is written as ZXG, how is BIG written? (a) YRT (b) YQT (c) XRT (d) YRS
A1 → Z26, C3 → X24, T20 → G7. Each pair adds to 27, so these are opposite letters. B → Y, I → R, G → T. Answer: (a) YRT.

Q4. If CAT = 24 and DOG = 26, what is BEE? (a) 10 (b) 12 (c) 13 (d) 15
CAT: 3 + 1 + 20 = 24. DOG: 4 + 15 + 7 = 26. Two examples confirm the sum-of-positions rule. BEE: 2 + 5 + 5 = 12. Answer: (b) 12.

Q5. If ROSE is coded as 6821, CHAIR as 73456 and PREACH as 961473, how is SEARCH coded? (a) 241673 (b) 214763 (c) 214673 (d) 214637
Read off each letter's digit: R6 O8 S2 E1 from ROSE; C7 H3 A4 I5 R6 from CHAIR (R6 agrees); P9 from PREACH, whose other letters agree. SEARCH is S2 E1 A4 R6 C7 H3. Answer: (c) 214673.

Q6. In a code, "go home now" is "ta pi ro", "come home soon" is "ki pi lu", and "go there soon" is "ro mo lu". What is the code for "now"? (a) ta (b) ro (c) pi (d) lu
"home" is in sentences 1 and 2; the common code is pi. "go" is in 1 and 3; the common code is ro. In sentence 1 that leaves "now" = ta. Answer: (a) ta.

Substitution and decoding

In substitution questions, the answer is the new name of the correct thing. If "sky" is called "sea" and "sea" is called "land", then fish live in the sea, which is called "land". Find the real answer first, then look up what it is called.

Decoding reverses the rule. If the code is "+2 on each letter", decoding is "−2". If the rule was reversal plus shift, undo the shift first, then reverse.

Common traps

Practice

1. If FISH is written as GJTI, how is BIRD written? (a) AJSE (b) CHSE (c) CJSE (d) CJRE
2. If CAMEL is written as DZNDM, how is HORSE written? (a) INTRF (b) IPSTF (c) GNRSD (d) INSRF
3. If MANGO is written as OGNAM, how is APPLE written? (a) EPLPA (b) ELPPA (c) ALPPE (d) ELPAP
4. If DOG is written as WLT, how is CAT written? (a) XZG (b) YZG (c) XAG (d) XZH
5. If ANT = 35 and BEE = 12, what is RAT? (a) 37 (b) 38 (c) 39 (d) 40
6. If "sky" is called "sea", "sea" is called "land" and "land" is called "air", where do fish live? (a) Sky (b) Sea (c) Air (d) Land
7. If LAMP is coded as 5381 and PALM as 1358, what is the code for MAP? (a) 831 (b) 813 (c) 381 (d) 853
8. If WORD is written as VNQC, how is SPEAK written? (a) TQFBL (b) RODZJ (c) ROEZJ (d) RODAJ

Answers:

1. (c) CJSE. Each letter +1.
2. (d) INSRF. +1, −1, +1, −1, +1: H→I, O→N, R→S, S→R, E→F.
3. (b) ELPPA. The code is the word written backwards.
4. (a) XZG. Opposite letters: C3 → X24, A1 → Z26, T20 → G7.
5. (c) 39. Sum of positions (1 + 14 + 20 = 35, 2 + 5 + 5 = 12). RAT = 18 + 1 + 20 = 39.
6. (d) Land. Fish live in the sea, and the sea is called "land".
7. (a) 831. L5 A3 M8 P1 from LAMP; PALM = 1, 3, 5, 8 agrees. MAP = 8, 3, 1.
8. (b) RODZJ. Each letter −1; A goes back to Z.

What to do next

• Solve 25 coding questions a day for a week, writing the change under each letter.
• Make a one-line note of each rule type you meet; after a week, see which ones you still hesitate on.
• Revise the alphabet test until positions are instant, because every rule here depends on them.
• Then try mathematical operations, which apply the same symbol-substitution idea to numbers.