Coding - Decoding
🔒 Log in to trackSubstitution and sentence coding
🔒 Log in to trackTwo formats, no arithmetic:
- Object renaming: 'sky is called sea, sea is called water...' — answer from the renaming chain, never from reality. If fish live in the sea and sea is called water, fish live in water.
- Sentence code matching: 'red light = pa ka', 'green light = ka na' — align words to code-words across sentences. Words repeated across sentences keep the same code-word; eliminate to decode singletons.
Method for sentence codes: list code-words per sentence side by side; the shared word's code appears in both lists; whatever remains is unique by elimination.
Detailed notes
What is substitution coding?
The exam renames things or renames code words, and you must answer from the renamed world. Two very different looks share this subtopic:
- 'Is called' chains. "If cloud is called rain, rain is called wind..." — the real thing and its new name are linked in a chain, and you answer with the new name.
- Word-code matching. "In a code language, 'red light' is 'pa ka'..." — English words are written with code words (pa, ka, ni ...), usually jumbled, and you find which code stands for which word.
'Is called' chains — the one rule
The question always asks about the real world, and the answer lies in the renamed world.
- Ask: what is the real answer in everyday life? (What do farmers wait for? Rain.)
- Then look up what that thing is called in the question. (Rain is called wind.)
- Answer with the new name: wind.
Read the chain in one direction only. "Cloud is called rain" tells you the new name of cloud — it does not tell you the new name of rain. That comes from the next line of the chain.
Word-code matching — intersect and subtract
- A word common to two sentences has its code among the code words common to those two sentences.
- Find a code for every common word first. Then remove those codes from each sentence: what is left over belongs to the left-over words.
Example: 'red light' = 'pa ka', 'green light' = 'ka na'. 'light' is common and 'ka' is the common code → light = ka. In sentence 1 the leftover word 'red' takes the leftover code pa.
The 'cannot be determined' rule
Sometimes two codes remain possible for one word. Example: 'red rose' = 'pa ka', 'white lotus' = 'za ka'. No word is common to the two sentences, so 'ka' is not pinned to anything: 'rose' could be 'pa' or 'ka'. When two or more codes remain equally possible after all elimination, the answer is cannot be determined. Do not guess one of them — the exam keeps this option exactly for this case.
With three sentences, elimination usually pins every word down. Work in this order:
- Words common to two (or three) sentences.
- Codes common to the same sentences.
- Subtract fixed codes; assign leftovers.
Other substitution forms
- Longer chains with reverse look-up: "when do we sleep?" — real answer 'night' — night is called 'morning' → answer 'morning'.
- Digits or symbols standing for words: a small dictionary is given (1 = come, 2 = home ...); build the answer phrase in the same word order as the question asks.
Quick revision
- 'Is called': find the real-world answer first, then answer with its new name. Never mix the two worlds.
- One line of a chain names only the word at its left end.
- Word-code: common word ↔ common code; then subtract what is fixed.
- Two possible codes left after elimination → cannot be determined.
- Phrase codes keep the order in which the question writes the phrase.
Types of questions asked
Every way this subtopic shows up in exams — how to recognise it, the formula or logic to use, and a solved example.
Type 1: Simple 'is called' chainsvery common2 practice Q
Lines like 'if white is called blue, blue is called red ...' ending with a real-life question.
- Answer the everyday question with the REAL thing (what do we write with? a pen).
- Read off the new name of that real thing from the chain.
- Give the new name as the answer.
Why it works: the chain only renames things; the asked fact lives in the real world, the answer in the renamed one.
Example: If cloud is called rain, rain is called wind, wind is called sun — what do farmers wait for?
Farmers wait for real rain. Rain is called wind. Answer: Wind (not rain — that is cloud's new name).
Type 2: Word-code matching by eliminationcommon2 practice Q
Two or three English sentences with code words ('pa ka ni'); the code order is usually jumbled.
- Find words common to two sentences; their codes are the common code words.
- Fix those codes, then subtract them from each sentence.
- Left-over words take left-over codes.
Why it works: a word's code must appear in every sentence that contains the word.
Example: 'hot tea' = 'na pi', 'cold tea' = 'pi ka', 'hot milk' = 'na ru' — code for 'milk'?
hot ↔ na (sentences 1, 3), tea ↔ pi (1, 2). Sentence 2 leaves cold = ka; sentence 3 leaves milk = ru.
Type 3: Cannot be determined (two codes left)common2 practice Q
A word appears in only one sentence, and its sentence shares two or more codes with the other sentence.
- Fix every code you can by the common-word method.
- If two codes still fit the asked word, stop — the answer is 'cannot be determined'.
- Never pick one of the two possible codes.
Why it works: elimination needs leftovers of size one; size two leaves a genuine ambiguity.
Example: 'red rose' = 'pa ka', 'white lotus' = 'za ka' — what is the code for 'rose'?
No word is common to the two sentences, so 'ka' belongs to nothing for certain. Rose can be 'pa' or 'ka' → cannot be determined.
Type 4: Longer chains and reverse look-upcommon3 practice Q
Chains of four or five renames, or questions that run the chain the other way ('what does 'morning' really mean?').
- Write the chain as a list: real thing → new name.
- Start from the real-world fact the question asks about.
- Walk to its new name (or, for reverse look-up, find which real thing maps to the given name).
Why it works: a written-down chain stops the confusion that costs marks here.
Example: If day is called night, night is called morning, morning is called evening — when do we sleep?
We sleep at real night. Night is called morning. Answer: Morning.
Shortcut tricks
⚡ Elimination table
Write each sentence and its code as two aligned rows. Common words are common code-words — tick them. The word asked about then either appears directly or is the lone unmatched item.
Example: Q. 'red light' = pa ka; 'green light' = ka na. Code for red?
Sol. 'light' is common to both sentences → light = ka. In sentence 1, red is the other word → red = pa.
Cross out matched pairs; the answer is whatever is left unpaired.
Where students lose marks
Answering from real-world knowledge ('fish live in water') instead of the renaming chain.
Assuming code-word order matches word order — sentence codes often reverse it; alignment is by elimination, not position.
Missing a repeated word because of a plural/singular change (light vs lights) — check word stems.
Practice sets — 13 questions
Sets of 10, mixed across the question types above. Each answer comes with a step-by-step explanation.
Topic test · 13 questions
Suggested time 8 min · wrong answers go to your mistake notebook automatically.