Coding-Decoding
CSAT · Logical Reasoning
Introduction
Coding-decoding questions hide a rule that turns words into letters or numbers. Write the alphabet positions once, compare the sample word with its code letter by letter, and the rule usually shows itself within a minute.
Key concepts
Alphabet positions
A1 B2 C3 D4 E5 F6 G7 H8 I9 J10 K11 L12 M13 N14 O15 P16 Q17 R18 S19 T20 U21 V22 W23 X24 Y25 Z26. Reverse positions: A26 … Z1. Memory aid EJOTY = 5, 10, 15, 20, 25.
Common rules
| Rule | Example |
|---|---|
| Shift each letter by k | CAT → DBU (+1) |
| Reverse the word, then shift | CAT → TAC → UBD |
| Opposite letters (sum of positions 27) | A ↔ Z, B ↔ Y, M ↔ N |
| Alternate shifts | +1, −1, +1 … |
| Letter positions to numbers | CAT → 3, 1, 20 or their sum 24 |
| Word substitution | 'sky' is called 'sea', 'sea' is called 'land' … |
Sentence codes
If 'pit na sa' means 'you are good' and 'na ta da' means 'are we well', the common word 'are' has the common code 'na'. Compare pairs of sentences to isolate each word.
Method
- Write the positions of the sample word and its code.
- Find the difference letter by letter – a constant difference means a shift.
- Check reversal if the differences look random.
- Apply the rule to the new word and match the options.
- Opposite letter = 27 − position
- EJOTY = 5, 10, 15, 20, 25
- Shift: position + k (wrap after 26)
Solved examples
Example 1
In a certain code language, FIELD is written as CKDHE. How is MONEY written in that code?
(a) YENOM (b) XDMNL (c) LNMDX (d) WCLMK
- Compare FIELD with CKDHE: the word is reversed and each letter moved 1 place back.
- Apply the same rule to MONEY: XDMNL.
Answer: (b) XDMNL
Example 2
In a certain code language, CLOUD is written as 55. How is SUGAR written in that code?
(a) 66 (b) 71 (c) 67 (d) 64
- Compare CLOUD with 55: the code is the sum of the alphabet positions of the letters (A = 1 … Z = 26).
- Apply the same rule to SUGAR: 66.
Answer: (a) 66
Example 3
In a certain code language, STONE is written as 73. How is WATER written in that code?
(a) 72 (b) 67 (c) 68 (d) 65
- Compare STONE with 73: the code is the sum of the alphabet positions of the letters (A = 1 … Z = 26).
- Apply the same rule to WATER: 67.
Answer: (b) 67
Example 4
In a certain code language, MANGO is written as PDQJR. How is TRAIN written in that code?
(a) WUDLQ (b) VTCKP (c) QLDUW (d) XVEMR
- Compare MANGO with PDQJR: each letter is moved 3 places forward in the alphabet.
- Apply the same rule to TRAIN: WUDLQ.
Answer: (a) WUDLQ
Example 5
In a certain code language, CHAIR is written as BGZHQ. How is TRAIN written in that code?
(a) TRAIN (b) RPYGL (c) MHZQS (d) SQZHM
- Compare CHAIR with BGZHQ: each letter is moved 1 place back in the alphabet.
- Apply the same rule to TRAIN: SQZHM.
Answer: (d) SQZHM
Example 6
In a certain code language, NORTH is written as ONSSI. How is DREAM written in that code?
(a) NZFQE (b) DPEYM (c) FRGAO (d) EQFZN
- Compare NORTH with ONSSI: letters in odd positions move +1 and letters in even positions move −1.
- Apply the same rule to DREAM: EQFZN.
Answer: (d) EQFZN
Example 7
In a certain code language, TIGER is written as GRTVI. How is FIELD written in that code?
(a) URVOW (b) TQUNV (c) VSWPX (d) WOVRU
- Compare TIGER with GRTVI: each letter is replaced by its opposite letter (A↔Z, B↔Y …, position p ↔ 27 − p).
- Apply the same rule to FIELD: URVOW.
Answer: (a) URVOW
Example 8
In a certain code language, BRAIN is written as MHZQA. How is STONE written in that code?
(a) RSNMD (b) CLMRQ (c) DMNSR (d) ENOTS
- Compare BRAIN with MHZQA: the word is reversed and each letter moved 1 place back.
- Apply the same rule to STONE: DMNSR.
Answer: (c) DMNSR
- Check just the first and last letters of the options – usually enough to eliminate three.
- For sentence codes, look for words common to two sentences.
- Write the alphabet with positions in the margin at the start of the exam.
- Forgetting to wrap around after Z.
- Applying a forward shift where the code moves backward.
- Assuming the codes in a sentence appear in the same order as the words.
Practice set
Attempt all 12 questions (about 90 seconds each), then check the worked solutions below.
Q1. In a certain code language, PLANT is written as 63. How is FRUIT written in that code?
(a) 75 (b) 74 (c) 72 (d) 79
Q2. In a certain code language, TIGER is written as UHHDS. How is CROWN written in that code?
(a) ERQWP (b) CPOUN (c) DQPVO (d) OVPQD
Q3. In a certain code language, MONEY is written as NNODZ. How is CHAIR written in that code?
(a) EHCIT (b) SHBGD (c) CFAGR (d) DGBHS
Q4. In a certain code language, SUGAR is written as TTHZS. How is FIELD written in that code?
(a) EKFHG (b) FGEJD (c) GHFKE (d) HIGLF
Q5. In a certain code language, SMILE is written as TLJKF. How is FRUIT written in that code?
(a) GQVHU (b) FPUGT (c) UHVQG (d) HRWIV
Q6. In a certain code language, TABLE is written as UZCKF. How is FRUIT written in that code?
(a) GQVHU (b) UHVQG (c) FPUGT (d) HRWIV
Q7. In a certain code language, FRUIT is written as SHTQE. How is NORTH written in that code?
(a) HTRON (b) GSQNM (c) FRPML (d) MNQSG
Q8. In a certain code language, SMILE is written as FMJNT. How is CLOUD written in that code?
(a) DMPVE (b) DUOLC (c) FWQNE (d) EVPMD
Q9. In a certain code language, TIGER is written as 59. How is TRAIN written in that code?
(a) 60 (b) 63 (c) 67 (d) 62
Q10. In a certain code language, PEARL is written as 52. How is STONE written in that code?
(a) 73 (b) 74 (c) 78 (d) 71
Q11. In a certain code language, CHAIR is written as DIBJS. How is PLANT written in that code?
(a) PLANT (b) UOBMQ (c) QMBOU (d) RNCPV
Q12. In a certain code language, TRAIN is written as GIZRM. How is EARTH written in that code?
(a) WAJHT (b) VZIGS (c) SGIZV (d) UYHFR
Q1 – (b) 74. Compare PLANT with 63: the code is the sum of the alphabet positions of the letters (A = 1 … Z = 26). Apply the same rule to FRUIT: 74.
Q2 – (c) DQPVO. Compare TIGER with UHHDS: letters in odd positions move +1 and letters in even positions move −1. Apply the same rule to CROWN: DQPVO.
Q3 – (d) DGBHS. Compare MONEY with NNODZ: letters in odd positions move +1 and letters in even positions move −1. Apply the same rule to CHAIR: DGBHS.
Q4 – (c) GHFKE. Compare SUGAR with TTHZS: letters in odd positions move +1 and letters in even positions move −1. Apply the same rule to FIELD: GHFKE.
Q5 – (a) GQVHU. Compare SMILE with TLJKF: letters in odd positions move +1 and letters in even positions move −1. Apply the same rule to FRUIT: GQVHU.
Q6 – (a) GQVHU. Compare TABLE with UZCKF: letters in odd positions move +1 and letters in even positions move −1. Apply the same rule to FRUIT: GQVHU.
Q7 – (b) GSQNM. Compare FRUIT with SHTQE: the word is reversed and each letter moved 1 place back. Apply the same rule to NORTH: GSQNM.
Q8 – (d) EVPMD. Compare SMILE with FMJNT: the word is reversed and each letter moved 1 place forward. Apply the same rule to CLOUD: EVPMD.
Q9 – (d) 62. Compare TIGER with 59: the code is the sum of the alphabet positions of the letters (A = 1 … Z = 26). Apply the same rule to TRAIN: 62.
Q10 – (a) 73. Compare PEARL with 52: the code is the sum of the alphabet positions of the letters (A = 1 … Z = 26). Apply the same rule to STONE: 73.
Q11 – (c) QMBOU. Compare CHAIR with DIBJS: each letter is moved 1 place forward in the alphabet. Apply the same rule to PLANT: QMBOU.
Q12 – (b) VZIGS. Compare TRAIN with GIZRM: each letter is replaced by its opposite letter (A↔Z, B↔Y …, position p ↔ 27 − p). Apply the same rule to EARTH: VZIGS.
- Positions + EJOTY
- Compare letter by letter
- Reverse? shift? opposite?
- Sentence codes: common word ↔ common code
