Coding-Decoding

Coding-decoding is a staple of AFCAT reasoning. Words or numbers are encoded by a hidden rule; you crack the rule and apply it. The single most useful tool is knowing the alphabet positions cold.

Know your alphabet positions

A=1, B=2, ... Z=26. Reverse: Z=1 ... A=26 (position from the back = 27 - forward position). Mnemonic EJOTY: E=5, J=10, O=15, T=20, Y=25 - anchors to count quickly.

Common coding types

• Letter shift: each letter moves a fixed number. If CAT = DBU (each +1), then DOG = EPH.
• Reverse coding: A<->Z, B<->Y (opposite letters, positions sum to 27).
• Number coding: letters replaced by their position (A=1) or by position +/- k.
• Substitution coding: whole words are swapped ("sky is blue" -> "blue is red" style); track which word maps to which.

Exam Tricks & Tips

• 🎯 Write A-Z with numbers 1-26 in the margin at the start; it speeds every coding question.
• 🎯 Use EJOTY (5,10,15,20,25) to jump to a letter's position without counting from A.
• 🎯 For reverse coding, opposite letters sum to 27 (A+Z, B+Y, C+X).
• 🎯 Find the shift by comparing one known letter pair, then apply it to the rest.
• 🎯 In substitution coding, list the word-to-word mapping before answering.
• ❌ Common mistake: miscounting the shift by including the start letter - "A to D" is a shift of 3, not 4; count the gaps, not the letters.

Quick recap

Memorise alphabet positions and the reverse (sum 27), use EJOTY to count fast, find the fixed shift from one pair, and track word mappings in substitution. Coding-decoding is quick once positions are automatic.

Cover the answer, recall, then check. 11 coding-decoding cards for AFCAT.

Q1. What is the position of the letter T? (use EJOTY)
A1. 20 (E=5, J=10, O=15, T=20, Y=25).

Q2. In reverse order, opposite letters sum to what?
A2. 27 (A+Z, B+Y, C+X, ...).

Q3. If CAT = DBU, decode the rule and encode DOG.
A3. Each letter +1, so DOG = EPH.

Q4. Which mnemonic anchors alphabet positions?
A4. EJOTY — E5, J10, O15, T20, Y25.

Q5. The opposite letter of C is?
A5. X (since 3 + 24 = 27).

Q6. What is the shift from A to D?
A6. 3 (count the gaps, not the letters).

Q7. Position of M and N?
A7. M = 13, N = 14 (the middle of the alphabet).

Q8. First thing to write down for coding questions?
A8. The alphabet A-Z with positions 1-26.

Q9. How do you find a letter-shift code's rule?
A9. Compare one known letter pair to get the fixed shift, then apply it.

Q10. In substitution coding, what should you list first?
A10. The word-to-word mapping.

Q11. Common coding mistake?
A11. Miscounting the shift by including the starting letter.

Coding-decoding hands you a made-up cipher and asks you to encode or decode a new word. It looks cryptic but is really pattern-matching: figure out how the given code was formed, then apply the same rule. A little alphabet fluency makes it fast and almost error-proof.

Core idea / what this tests: whether you can deduce the rule linking a word to its code — letter shifts, reversals, position numbers or substitutions — and apply it to a fresh word.

Deep explanation

Beginner — know your alphabet positions

Most codes use letter positions, so memorise anchors: A=1, E=5, J=10, O=15, T=20, Z=26. Also learn opposite pairs (A↔Z, B↔Y, C↔X...), which sum to 27 — the "EJOTY" trick (E=5, J=10, O=15, T=20, Y=25) lets you locate any letter in seconds.

Intermediate — the common coding types

Type	Rule	Example
Letter shift	+n or −n along alphabet	CAT → DBU (+1)
Reverse/opposite	replace with A↔Z pair	CAT → XZG
Positional number	letters → their numbers	CAT → 3-1-20
Letter jumble	rearrange letters	CAT → ATC
Substitution	word ↔ another word	"sky is blue" style

Start by comparing the word and its code letter-by-letter to spot the shift or pattern.

Advanced — substitution and matrix codes

• Substitution coding ("if pen = table, table = book, what do you write with?") ignores real meaning — track only the substitutions, answering with the coded word.
• Number-word coding with multiple sentences: find words common to two coded lines and match them to the common code number to crack each symbol.
• Watch for mixed rules (shift the vowels one way, consonants another) — test a second given example to confirm the pattern before applying it.

Worked example

Q. If in a certain code FLIGHT is written as GMJHIU, how is PILOT written?

Compare F→G, L→M, I→J, G→H, H→I, T→U — each letter shifts +1. Apply +1 to PILOT: P→Q, I→J, L→M, O→P, T→U = QJMPU.

Answer: QJMPU. Detect the shift on the given pair, then apply it uniformly — the standard coding move.

AFCAT relevance

Coding-decoding is a regular, self-contained scorer in AFCAT reasoning. It needs no outside knowledge — just the deduced rule — so with alphabet fluency it is fast and reliable, and the systematic approach guards against slips under negative marking.

Speed tricks and shortcuts

• Memorise EJOTY (5,10,15,20,25) to find letter positions instantly.
• Opposite letters sum to 27 (A+Z, B+Y) for reverse codes.
• Compare word and code letter-by-letter to reveal the shift.
• Confirm the rule on a second example before coding the target.

Worked Example

Problem: In a certain code language, CAT is written as DBU. How is DOG written in the same code?

Solution:
Compare each letter of the plain word with its coded letter to find the rule.
CAT → DBU:

• C (3rd letter) → D (4th letter): +1 position.
• A (1st) → B (2nd): +1 position.
• T (20th) → U (21st): +1 position.

The rule is: shift each letter forward by 1 place in the alphabet.

Apply the same rule to DOG:

• D → E
• O → P
• G → H

So DOG is coded as EPH.

Answer: DOG is written as EPH.

Solution note: Always assign position numbers (A = 1, B = 2, …, Z = 26) to spot the shift precisely, and remember the alphabet "wraps around" (Z + 1 = A) in some codes.