Coding-Decoding

Coding-Decoding

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Coding-Decoding tests your ability to spot a hidden rule that links a code word to its original word (or vice versa). The rule is always applied letter-by-letter or digit-by-digit using the standard position value where A=1, B=2 … Z=26. Once the rule is cracked from the given example, applying it to the answer choice is mechanical.

• The four rule families you must recognise: letter-number substitution, fixed shift (forward or backward), reverse alphabet (A↔Z, B↔Y …), and operation-based (+, −, ×, ÷ on digits).
• Always verify the rule on every letter of the example pair, not just two letters.
• In NAT-I, expect 1–3 MCQs worth 2–4 marks total — solve in under 90 seconds each.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Core rule families

Every NAT-I coding item hides one of these operations. The candidate’s job is to identify which family the example pair belongs to and then mirror the same operation on the target word.

Rule family	How it works on a letter	Sample: original → coded
Position value (A=1…Z=26)	Replace each letter by its serial number	CAT → 3, 1, 20
Forward shift (+k)	Move k places right in the alphabet	CAT (+1) → DBU
Backward shift (−k)	Move k places left	CAT (−1) → BZS
Reverse alphabet	Mirror: position becomes 27−position	CAT (27−pos) → XZG
Operation on digits	Apply +, −, ×, ÷ to numeric digits	468 → 4+6, 4×6 → 10, 24

Worked example (letter-number substitution)

If CAT = 3-1-20 and DOG = 4-15-7, then PIG = 16-9-7. The rule is simply “write the position value of each letter separated by a hyphen”. Any word becomes a string of digits 1 to 26.

Standard exam patterns

• Single-rule items: one example pair, one answer word — fastest to solve.
• Mixed-rule items: letters shifted and digits operated on in the same word (e.g., +2 on letters, sum of digits on numbers).
• Analogy items: A is to B as C is to ? — same rule linking both pairs.

Trap alert: NAT-I options are designed so that a half-applied rule (e.g., shifting only the first letter) still produces a plausible word. Always test your rule on the last letter of the example pair before committing.

Common mistakes to avoid

• Confusing forward shift with reverse — a shift of −1 on C gives B, not Z.
• Treating A as 0; NAT-I uses the 1-indexed A=1 convention.
• Reversing coded and original — if the question asks for the original, your answer is a real English word, not a digit string.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Edge cases and deeper mechanics

Most NAT-I items use a single rule, but higher-difficulty items stack two rules — for instance, a forward letter shift followed by writing each new letter as its position value (CAT +1 → DBU → 4-2-21). Others combine a letter rule on the first half of a word with a digit rule on the second half.

Number-coding items sometimes chain operations: square a digit, then add the digits of the square (4 → 16 → 1+6 = 7). Such chains almost always apply to every digit, which is how you verify them.

Worked micro-example

Given BOX → 2-15-24 and PEN → 16-5-14, find the code for NET.

1. Rule check on BOX: B=2, O=15, X=24 ✓
2. Rule check on PEN: P=16, E=5, N=14 ✓
3. Apply to NET: N=14, E=5, T=20 → 14-5-20

Answer: 14-5-20, with each digit matching its letter position. This is the canonical letter-number substitution question NAT-I repeats.

Connections to adjacent topics

• Series completion uses the same A=1 positional idea but in arithmetic progression.
• Analogy reasoning is the parent category — coding-decoding items are analogy items where both sides of the colon are strings rather than single objects.
• Direction sense occasionally appears coded as N-N-E-S strings; the alphabet-shift skill transfers directly.

Time-budget rule: spend under 90 seconds per coding MCQ. If the rule isn’t visible after checking letter positions A=1…Z=26 in both example words, mark and move — NAT-I rarely hides the rule deeper than that.

Practice prompts

1. If WIND → 23-9-14-4 and FIRE → 6-9-18-5, code WOLF.
2. If DOG (+3) → GRJ, decode CAT (+3) back to its original using a shift of −3.

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