Coding-Decoding: Logical Reasoning Notes for BITSAT

Coding‑Decoding – Logical Reasoning for BITSAT

(Beginner‑friendly notes, no jargon, everyday analogies, short sentences, tiny example)

1. What is Coding‑Decoding?

Coding	Decoding
Turning a normal message into a secret one.	Turning the secret back into the normal message.

Think of it like writing a note in a secret code so only you and your friend can read it.
When you decode, you read what your friend wrote back into plain English.

2. Everyday Analogy

Password: When you type a password, you code your password into a computer. The computer decodes it to check if it matches the stored one.
Morse Code: A dot‑dash rhythm stands for letters. The receiver decodes the rhythm back into words.
Secret Handshake: Two friends use a special sequence of moves. Only they decode the meaning of the handshake.

3. How to Solve a Coding‑Decoding Question

Look for Repeated Symbols
- If a symbol appears many times, it might stand for a common letter like E or T.
- Example: “X X Y” → the pair “X X” could be “S S” or “L L”.
Check the Length of Words
- A one‑letter word is almost always A or I.
- A two‑letter word could be AN, AT, OF, TO, IN, etc.
Find Common Patterns
- “A B A” → maybe “I * * I” or “A * A”.
- “C C C” → maybe “E E E” or “T T T”.
Guess and Test
- Pick the most likely letter for a symbol.
- Replace it in the whole message.
- See if the words look like real English.
Use Elimination
- If a symbol could be E or T, look at the context.
- If the word ends with that symbol and the word is “END” or “SET”, choose the matching letter.
Cross‑Check
- After filling some letters, check the whole sentence for sense.
- If it still looks odd, try a different letter for the ambiguous symbol.

4. Common Types of Coding in BITSAT

Type	What It Looks Like	Quick Hint
Substitution	Each letter replaced by a symbol (e.g., A → X, B → Y).	One‑to‑one mapping.
Transposition	Letters are shuffled but the same letters remain.	Same set of letters, order changed.
Mixed	Combination of substitution and transposition.	Solve substitution first, then shuffle.

5. Tiny Example

Encoded sentence:

X Y Z X

Step‑by‑step:

One‑letter word?
- No, the shortest word has 2 letters.
Pattern “X Y Z X”
- The first and last letters are the same → could be “I _ _ I” or “A _ _ A”.
Common 2‑letter words
- “in”, “on”, “to”, “of”.
Try “I” for X
- Sentence becomes I _ _ I.
Try “ON” for Y Z
- Y = O, Z = N → I O N I → “I ON I” → nonsense.
Try “A” for X
- Sentence becomes A _ _ A.
Try “TO” for Y Z
- Y = T, Z = O → A T O A → “A TO A” → nonsense.
Try “TO” for Y Z again with different mapping
- Y = O, Z = T → A O T A → “A OT A” → nonsense.
Try “OF” for Y Z
- Y = O, Z = F → A O F A → “A OF A” → nonsense.
Try “ON” for Y Z
- Y = O, Z = N → A O N A → “A ON A” → nonsense.

Realisation:
Maybe it’s a transposition: letters X Y Z X are just shuffled. The actual word could be “ANNA” (a name).
Mapping: X = A, Y = N, Z = N → A N N A → “ANNA”.

Decoded sentence:
ANNA

(In a real BITSAT question, you’ll get more context or a longer sentence, so the process is quicker.)

6. Quick Practice Question

Encoded:
B D B C
What does it read?

Hint: One‑letter words and common two‑letter words.

Answer:
B D B C → “I * * I” (B = I).
The middle two letters could be “TO” → D = T, C = O.
Decoded: I T O I → “I TO I” → not good.
Try “ON”: D = O, C = N → I O N I → “I ON I”.
Try “OF”: D = O, C = F → I O F I →