Coding-decoding tests your ability to detect rule-based patterns and transformation rules used to encode words, numbers, or sentences. Deciphering letter, number, and substitution codes requires analyzing positional shifts, reverse alphabetical pairs, jumbled structures, and direct symbol assignments.

Alphabetical positions and key rules

Mastering letter positions from A to Z ($1$ to $26$) is essential for quick pattern identification.

• EJOTY Rule: Positions of E ($5$), J ($10$), O ($15$), T ($20$), and Y ($25$) serve as benchmark reference points.
• CFILORUX Rule: Multiples of $3$: C ($3$), F ($6$), I ($9$), L ($12$), O ($15$), R ($18$), U ($21$), and X ($24$).
• Reverse Positional Value: Reverse rank = $27 -$ Forward rank. For instance, Z is $27 - 26 = 1$.
• Opposite Letter Pairs: Pairs that sum to $27$, such as 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.

Major categories of coding

1. Letter Coding

Letters of a word are replaced by other letters using specific transformation patterns:

• Forward / Backward Shift: Adding or subtracting a fixed or incremental number to each letter's rank (e.g., $+1, +2, +3$ or $-2, -2, -2$).
• Reverse Order / Jumbling: Reversing the whole word or splitting the word into halves and reversing each half.
• Opposite Pair Coding: Replacing each letter with its complementary pair from the opposite end of the alphabet.

2. Number and Symbol Coding

Words are represented by digits or special symbols:

• Direct Position Sum: Summing positional values of all letters (e.g., CAT = $3 + 1 + 20 = 24$).
• Reverse Position Sum: Summing reverse values ($27 -$ Forward rank).
• Direct Character Assignment: Assigning fixed digits/symbols to specific characters directly shared across given word pairs.

3. Substitution Coding

Words are substituted with fictitious names in a sequence (e.g., "red is called blue, blue is called green..."). Look for the object in question and pick the name it is directly called or substituted with.

4. Mixed Message / Sentence Coding

Multiple full phrases and their corresponding coded statements are provided. By comparing statements pairwise, common words and common code tokens are identified through elimination.

A reliable solving method

1. Write down positional numbers above the original and coded words.
2. Check for direct letter shifts ($+n / -n$) or position reversal.
3. If letters remain unchanged in set composition, inspect reordering/jumbling patterns.
4. For number codes, calculate letter position sums (forward and reverse).
5. For mixed sentence coding, compare two statements at a time to isolate unique word-code pairs.

Common mistakes

• Confusing "means" with "is called" in substitution problems.
• Miscounting letter shifts around boundary transitions (e.g., moving from Z to A or A to Z).
• Calculating forward position sum when the pattern requires reverse position sum.
• Assuming fixed alphabetical shift without checking all letters in the word.

Quick revision summary

• Reverse rank = $27 -$ Forward rank.
• Opposite letter pair sum = $27$.
• In "A is called B", answer for A is B. In "A means B", answer for B is A.
• Cross-compare common words in sentence coding to eliminate options.