Coding-decoding: Reasoning Ability Notes for NABARD Development Assistant

Coding‑Decoding – Study Notes

(NABARD Development Assistant – Reasoning Ability)

1. Simple Explanation

Coding‑decoding problems present a sequence of words (or numbers) and a rule that transforms one word into the next.
Your job is to:

Identify the rule (letter shifts, pattern of positions, numeric mapping, etc.).
Apply the rule to determine the missing element(s) or to find the next term in the sequence.

The key is to look for consistent operations (e.g., “replace each letter by the next one in the alphabet”, “reverse the word”, “write the word in reverse order and then change the middle letter”, etc.) and to test the rule on all given terms.

2. Key Points (What to Watch For)

#	What to Look For	Typical Patterns
1	Alphabetic shifts	A→B, B→C, ...; or cyclic shifts (Z→A).
2	Position‑based changes	Change only the first, last, or middle letter.
3	Reversal	The next word is the reverse of the previous one.
4	Letter deletion/duplication	Remove or duplicate certain letters.
5	Transposition	Swap adjacent letters or move a letter to a new position.
6	Numeric mapping	Convert letters to numbers (A=1, B=2, …) and apply arithmetic.
7	Combination of the above	Eg. reverse + shift, or shift + delete.
8	Consistent rule across all pairs	A valid rule must hold for every consecutive pair in the series.
9	Check both directions	Sometimes the rule is applied forward or backward.
10	Look for wordplay	Hidden meanings, homophones, or palindromes.

3. Worked Example

Series:

GREECE → TIYVEB → BUBNIK → ? → WIKXKD

Step 1: Identify the rule

GREECE → TIYVEB
- G→T (shift +13, i.e., reverse alphabet)
- R→I (shift -9)
- E→Y (shift +20)
- E→V (shift +17)
- C→E (shift +2)
- E→B (shift -3)
→ Not a simple uniform shift.
→ Check if letters are reversed:
G R E E C E → E C E E R G (reverse).
Compare with TIYVEB: no.
Look for a pattern of “shift by position index”
- Position 1: G→T (+13)
- Position 2: R→I (−9)
- Position 3: E→Y (+20)
- Position 4: E→V (+17)
- Position 5: C→E (+2)
- Position 6: E→B (−3)
These shifts are alternating + / − but not consistent.
Try mapping each letter to its counterpart in the alphabet (A↔Z, B↔Y, ...) – the Atbash cipher:
- G (7) → T (20) ✔
- R (18) → I (9) ✔
- E (5) → V (22) ✔
- E (5) → V (22) ✔
- C (3) → X (24) ❌ (expected X but we see E)
So not exact Atbash.
Experiment with “reverse + Atbash”:
- Reverse GREECE → ECEERG.
- Apply Atbash:
  - E→V
  - C→X
  - E→V
  - E→V
  - R→I
  - G→T → gives VXVVIT (not TIYVEB).
Try “reverse each word and then shift letters by +1”:
- GREECE → ECEERG → shift +1 → FDFFHS → no.
Check for “alternating Atbash and simple shift”:
Maybe each letter is transformed differently:
- G→T (Atbash)
- R→I (Atbash)
- E→Y (shift +20)
- E→V (shift +17)
- C→E (shift +2)
- E→B (shift -3)
Still arbitrary.

Better approach: Look at the second pair:

TIYVEB → BUBNIK

T→B (shift +8 or -18)
I→U (+12)
Y→B (+8)
V→N (-10)
E→I (+4)
B→K (+9)

The shifts here are not uniform.

Observation: The series might be built on a two‑step process:

Apply a fixed cipher to the word.
Then reverse the result.

Test this:

Take GREECE → apply Atbash → TIVVIT.
Reverse → TIVVIT (palindrome).
But TIYVEB is not.

Conclusion: The series is too irregular for a single simple rule.
In practice, for exam patterns, such a series would be discarded or you would look for a simpler sub‑pattern.

4. A Clear Example (Used in Exams)

Series:

BRAIN → NARAB → ? → MURAM

Goal: Find the missing word (fourth term).

Step‑by‑Step

Identify the rule
Observe that each word is the reverse of the previous one with the middle letter replaced by the next letter in the alphabet.
- BRAIN → reverse → NIARB
  Replace middle letter ‘I’ with next letter ‘J’ → NJARB
  But the second term is NARAB. So our assumption is wrong.
Try “reverse + delete middle letter”
- BRAIN → reverse → NIARB → delete middle (A) → NIRB – not NARAB.
**Check “reverse + change