SSC CGL Reasoning · Free notes
Coding-Decoding: SSC CGL Reasoning
Coding-Decoding has 247 questions in the SSC CGL Tier I bank, and 95 of them come from the 2025 shifts. Most are quick: 163 are EASY and only 5 HARD, so at about 36 seconds a question the marks go to a fixed method rather than to insight. Start every question the same way: write the example word above its code and ask what changed, the letters themselves or only their places. Know the alphabet positions by heart (EJOTY, and opposite letters adding to 27), because nearly every rule is counted on them. The marks are lost in three places: a shift that runs past Z without wrapping round, a reversal not undone before the gaps are counted, and a rule tested on one example when two are printed.
The handout has every subtopic, worked example, formula and trap with answers shown. The formula sheet is the chapter’s formulas and traps alone, on one page.
Subtopic notes
Letter Shifts
82 PYQsEach letter of the word moves a fixed number of places along the alphabet: one shift for every letter, a pattern of shifts that changes from letter to letter, or one shift for vowels and another for consonants.
Reversal and Opposite Letters
49 PYQsThe letters of the word are put in a new order (reversed, split into halves, swapped in pairs), often with a letter shift on top, or each letter is replaced by its opposite letter, the one in the same place from the other end of the alphabet.
Position Values and Sums
60 PYQsLetters become numbers: each letter is written as its place in the alphabet (from the front or from the back, sometimes changed by a fixed rule), the places are added into one number, or the code depends only on how many letters the word has.
Number and Symbol Coding
30 PYQsEach letter has its own digit or symbol: either the code keeps the letters' order and can be read off like a table, or the code is shuffled and one letter's sign is found from what two words share.
Word-Code Sentences
26 PYQsWhole sentences are coded word by word, with the codes in a mixed order; a word that two sentences share takes the code that their two codes share.
Formula & revision sheet
4 formulas · 1 reference tables · 29 gotchas across all subtopics — the exam-eve cheat-sheet
Formula & revision sheet
4 formulas · 1 reference tables · 29 gotchas across all subtopics — the exam-eve cheat-sheet
Watch out for (8)
- Forgetting to wrap round→ One shift for every letter
- Answering with the first code when the code is coded again→ One shift for every letter
- Checking only the first letter→ One shift for every letter
- Taking the first gap as the rule for every letter→ A pattern of shifts that changes from letter to letter
- Turning a backward gap into a forward one→ A pattern of shifts that changes from letter to letter
- Forcing a shape that is not there→ A pattern of shifts that changes from letter to letter
- Judging the letter after it has moved→ Vowels and consonants moved differently
- Calling it one shift when the vowels stayed→ Vowels and consonants moved differently
Reference tables (1)
Opposite letters: pairs that add to 2713 rows
| Letter | Position | Opposite | Opposite's position |
|---|---|---|---|
| A | 1 | Z | 26 |
| B | 2 | Y | 25 |
| C | 3 | X | 24 |
| D | 4 | W | 23 |
| E | 5 | V | 22 |
| F | 6 | U | 21 |
| G | 7 | T | 20 |
| H | 8 | S | 19 |
| I | 9 | R | 18 |
| J | 10 | Q | 17 |
| K | 11 | P | 16 |
| L | 12 | O | 15 |
| M | 13 | N | 14 |
Watch out for (7)
- Shifting letters that were only moved→ Reversing and rearranging the letters
- Reversing the whole word when only the halves were reversed→ Reversing and rearranging the letters
- Counting gaps before undoing the reversal→ Rearrange first, then shift
- Running a growing shift from the wrong end→ Rearrange first, then shift
- Stopping after one example fits→ Rearrange first, then shift
- Using 26 instead of 27→ Opposite letters: pairs that add to 27
- Hunting for a shift in an opposite code→ Opposite letters: pairs that add to 27
Formulas (3)
Watch out for (6)
- Splitting a joined code the wrong way→ Each letter written as a number
- Reversing the digits when the letters were reversed→ Each letter written as a number
- Trusting a step that fits one example→ Adding the letters into one number
- Counting a repeated letter once→ Adding the letters into one number
- Adding letters when only the length matters→ Codes from the number of letters, and number systems
- Dropping the zeros in binary→ Codes from the number of letters, and number systems
Watch out for (4)
- Assuming the code is in letter order→ Reading a code off letter by letter
- Ignoring a repeated letter→ Reading a code off letter by letter
- Giving a shared letter one digit→ One letter left over: matching the common letters
- Reading a shuffled code in letter order→ One letter left over: matching the common letters
Watch out for (4)
- Reading the codes in word order→ Two sentences: the shared word takes the shared code
- Picking one code when two words are shared→ Two sentences: the shared word takes the shared code
- Placing a shared code before setting aside the fixed ones→ Three or more sentences: fix one word, then the next
- Rejecting an option for the order of its codes→ Three or more sentences: fix one word, then the next
PYQ weightage by concept
13 concepts · 247 PYQs — where the marks actually sit, so you know what to drill first
PYQ weightage by concept
13 concepts · 247 PYQs — where the marks actually sit, so you know what to drill first
| Concept | PYQs | Share |
|---|---|---|
| One shift for every letter | 44 | 18% |
| A pattern of shifts that changes from letter to letter | 32 | 13% |
| Vowels and consonants moved differently | 6 | 2% |
| Concept | PYQs | Share |
|---|---|---|
| Rearrange first, then shift | 28 | 11% |
| Reversing and rearranging the letters | 17 | 7% |
| Opposite letters: pairs that add to 27 | 4 | 2% |
| Concept | PYQs | Share |
|---|---|---|
| Each letter written as a number | 32 | 13% |
| Adding the letters into one number | 19 | 8% |
| Codes from the number of letters, and number systems | 9 | 4% |
| Concept | PYQs | Share |
|---|---|---|
| One letter left over: matching the common letters | 20 | 8% |
| Reading a code off letter by letter | 10 | 4% |
| Concept | PYQs | Share |
|---|---|---|
| Three or more sentences: fix one word, then the next | 15 | 6% |
| Two sentences: the shared word takes the shared code | 11 | 4% |
Test yourself on Coding-Decoding
20 past SSC CGL questions from this chapter, timed at 12 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.