Coding Decoding Letter Number Substitution Questions for BIHAR POLICE CONSTABLE

Alphabet Position System (Foundation)

Every letter has a fixed position: A=1, B=2, C=3... Z=26. This is the backbone of 90% of coding problems. You must memorize this instantly. Think of it as a keyboard where each key has a number. Forward positions (A to Z) and reverse positions (Z=1, Y=2, X=3... A=26) are both common in exams.

• Forward: A=1, B=2, C=3... Z=26
• Reverse: Z=1, Y=2, X=3... A=26
• Middle letters: M=13, N=14 (useful for quick checks)
• Always write out A-Z with numbers on scratch paper in first 30 seconds of exam
• Reverse alphabet is trickier — use formula: Position_reverse = 27 - Position_forward

Direct Substitution (Simple Cipher)

One letter always maps to the same code throughout. The question gives you a coded word and asks you to decode it, or vice versa. The trick is identifying the pattern from the given example(s). Most common patterns: shift by fixed number, use position value, or reverse alphabet.

• Pattern is consistent across all letters in the problem
• Identify pattern from the given example (usually first coded word)
• Common shifts: +1 (A→B), -1 (A→Z), +5, +7, reverse alphabet
• Always verify your pattern with the second example if given
• Watch for position-based codes: A→1, B→2, etc.

Reverse Alphabet Substitution

Each letter is replaced by its reverse-alphabet counterpart. A↔Z, B↔Y, C↔X, etc. This is extremely common in Bihar Police exams. The reverse position formula (27 - position) is your best friend here. It looks complex but is mechanical once you practice.

• A=Z, B=Y, C=X, D=W, E=V, F=U, G=T, H=S, I=R, J=Q, K=P, L=O, M=N
• Mnemonic: 'A-Z, B-Y, C-X' — remember first three pairs, rest follows
• Apply formula 27 - pos for any letter instantly
• Reverse alphabet is self-inverse: if you apply it twice, you get original word
• Common in 'find the code' and 'find the word' both types

Position-Based & Arithmetic Codes

Letters are replaced by numbers (usually their position), then arithmetic operations (addition, subtraction, multiplication) are applied. These are trickier because they involve calculation. Watch for operations applied to all letters equally, or operations that vary by position.

• Simple position code: A=1, B=2, etc. (no operation)
• Position + constant: A=1+5=6, B=2+5=7, etc.
• Position × constant: A=1×2=2, B=2×2=4, etc.
• Position-dependent: 1st letter gets +1, 2nd gets +2, etc.
• Always check if result wraps around (e.g., Z+1 = A in some codes)

Word-to-Word Substitution (Complex Pattern)

Instead of letter-by-letter, the entire word or groups of letters follow a pattern. Less common but appears in harder questions. Requires identifying whether letters shift together, or if there's a word-level rule (e.g., reverse the word, rearrange by position).

• Check if word is reversed: HELLO → OLLEH
• Check if letters are rearranged by position: 1st↔last, 2nd↔2nd-last, etc.
• Check if each letter follows same rule as others (not word-specific)
• Identify from given example — usually only one pattern per question
• Verify with second example before applying to answer

• Forgetting the alphabet position values. Always write A=1 to Z=26 on paper immediately. Relying on memory leads to errors under pressure.
• Confusing forward and reverse alphabet. Z=26 (forward) but Z=1 (reverse). Read the question carefully — it will specify or hint via the example.
• Applying pattern inconsistently. If first letter shifts by +1, all letters must shift by +1. Don't assume different rules for different letters unless explicitly shown.
• Miscounting positions, especially near M-N (middle). Double-check M=13, N=14 as anchors.
• Ignoring the second example. Always verify your identified pattern against all given examples before answering. One mismatch means your pattern is wrong.

• ABCDEFGHIJKLMNOPQRSTUVWXYZ with numbers 1-26 written below — visualize this as your 'code key'. Draw it in 10 seconds at exam start.
• Reverse alphabet mnemonic: 'A-Z, B-Y, C-X, D-W, E-V, F-U, G-T, H-S, I-R, J-Q, K-P, L-O, M-N' — pairs are symmetric.
• Formula shortcut for reverse: 27 minus the position. (27 is one more than 26, so it's easy to remember.)
• Pattern check: 'Identify, Verify, Apply' — find pattern from first example, check with second, then apply to answer.

• Bihar Police Constable typically asks 2-3 coding-decoding questions in the Reasoning section. They are usually straightforward (not tricky) — focus on speed and accuracy, not complex logic.
• Most common types: reverse alphabet (40%), position value (30%), shift cipher (20%), arithmetic operations (10%). Prioritize reverse alphabet practice.
• Time allocation: 1-1.5 minutes per question. If you're stuck after 1 minute, skip and come back. These questions have no partial credit, so move on.
• Recent papers show preference for 'find the code' (word given, find code) over 'find the word' (code given, find word). Practice both, but 'find the code' is slightly more common.
• Watch for multi-word codes. Sometimes the question codes a phrase (e.g., 'HELLO WORLD'). Apply the pattern to each word separately — don't mix them.