Aptitude Reasoning: Coding and Decoding Types
Coding and Decoding is a critical topic in reasoning aptitude tests. It evaluates a candidate’s logical reasoning, pattern recognition, and ability to understand hidden relationships between elements (typically letters or numbers). In coding-decoding, a word, sentence, or a series of letters and numbers is coded in a particular way, and the objective is to decode it based on the given rules or find the code for a new word
Types of Coding-Decoding:
- Letter Coding
- Number Coding
- Substitution Coding
- Symbol Coding
- Mixed Letter-Number Coding
1. Letter Coding
“In letter coding, each letter in a word is replaced by another letter according to a certain rule, which may involve shifting letters forward or backward in the alphabet, reversing positions, or alternating positions.”
Example 1
If in a certain code, CAT → 24, what will DOG be coded as?
Solution pattern
- Assign alphabetical positions:
C=3, A=1, T=20 - Sum = 3 + 1 + 20 = 24
Now apply the same rule to DOG:
D=4, O=15, G=7
Sum = 4 + 15 + 7 = 26
✅ Answer: DOG → 26
Example 2
If in a certain code, BAT → 23, what will HEN be coded as?
Solution pattern
- BAT → B=2, A=1, T=20
Sum = 2 + 1 + 20 = 23
Now apply the same rule to HEN:
H=8, E=5, N=14
Sum = 8 + 5 + 14 = 27
✅ Answer: HEN → 27
2. Number Coding
“In number coding, each letter or word is assigned a specific number. The pattern may be based on alphabetical positions, addition or subtraction of digits, or a particular rule applied to each letter or to the entire word.”
Example 1
If in a certain code, the number 253 → 10, what will 481 be coded as?
Rule (Sum of digits)
2 + 5 + 3 = 10
Apply same rule to 481:
4 + 8 + 1 = 13
✅ Answer: 481 → 13
Example 2
If in a certain code, the number 734 → 14, what will 926 be coded as?
Rule (Sum of digits, then subtract 1)
7 + 3 + 4 = 14 → 14 − 0 = 14
(Here no change, but pattern is clear.)
Apply same rule to 926:
9 + 2 + 6 = 17 → 17 − 0 = 17
✅ Answer: 926 → 17
3. Substitution Coding
“In substitution coding, certain words are replaced with other words or numbers. The goal is to decode the sentence using these substitutions.”
Example 1
In a certain code language:
“Red” is coded as “Blue,”
“Blue” is coded as “Green,”
“Green” is coded as “Yellow.”
What is the color of the sky in that code language?
Solution
Normally, the sky is blue.
In the code language, Blue → Green.
✅ Answer: Green
Example 2
In a certain code language:
“Book” is written as “Paper,”
“Paper” is written as “Pen,”
“Pen” is written as “Ink.”
How will “Book” be written in that code language?
Solution
Book → Paper
✅ Answer: Paper
4. Symbol Coding
“In symbol coding, each symbol represents a number or letter. The code may include special characters (such as @, #, $, etc.), and understanding the rule requires mapping each symbol to its corresponding number or letter.”
Example 1
In a certain code language:
@ = 2, # = 5, $ = 8
What is the value of @ + $?
Solution
@ = 2
$ = 8
2 + 8 = 10
✅ Answer: 10
Example 2
In a certain code language:
% = 3, & = 4, *** = 7**
What is the value of % × * − &?
Solution
% = 3
- = 7
& = 4
3 × 7 = 21
21 − 4 = 17
✅ Answer: 17
5. Mixed Letter-Number Coding
“Mixed letter-number coding combines letters and numbers with a specific rule. The challenge often lies in figuring out how letters and numbers interact or follow a sequence. The underlying principle is the same as that of Letter Coding, where a hidden pattern must be decoded.”
Example 1
In a certain code language:
A = 1, B = 2, C = 3, …
Also, each vowel adds +2 to the total value of the word.
How is CAT coded?
Solution
C = 3
A = 1
T = 20
Sum = 3 + 1 + 20 = 24
A is a vowel → add +2
24 + 2 = 26
✅ Answer: CAT → 26
Example 2
In a certain code language:
Each consonant is assigned its alphabetical position.
Each vowel is assigned 5.
The code for a word is the product of all its letter values.
How is BAG coded?
Solution
B = 2
A = 5 (vowel rule)
G = 7
Product = 2 × 5 × 7 = 70
✅ Answer: BAG → 70
