Quantitative Aptitude · Number Series, Coding-Decoding and Odd Man Out

Coding-Decoding: Number and Substitution Coding

Updated 1 October 2026 · Fact-checked

Number coding replaces words or letters with numbers, often using alphabet positions (A=1 to Z=26) or arithmetic rules. Substitution coding swaps one item for another by a given rule. To solve, write the given code pairs, find the rule, test it on every example, then apply it to the question.

Understand Coding-Decoding: Number and Substitution Coding

Coding-decoding means a message is written in a secret form, and you must work out the rule. The question gives you one or more examples of the code. You use them to decode or encode a new word or number.

In number coding, letters or words become numbers. The common rules are: each letter is replaced by its position in the alphabet (A=1, B=2 ... Z=26), the positions are added or multiplied, or a fixed number is added to each position. Sometimes the digits are written side by side, and sometimes they are added to give one value.

In substitution coding, one item is simply replaced by another. For example, the question may say that 'red' is called 'blue' and 'blue' is called 'green'. Then you must answer using the new names. Here there is no arithmetic. The risk is losing track of which name belongs to which item.

The difference from letter coding is the output. Letter coding changes letters into other letters (by shifting or reversing). Number coding changes them into numbers. Substitution coding swaps whole items or names. Many questions mix these, so always check what the code actually produces.

The rule is always consistent. If your rule works for one example but fails for another, it is the wrong rule. Test it on every given example before you apply it.

Key formulas to remember

Alphabet position: A=1, B=2, C=3 ... M=13, N=14 ... Y=25, Z=26; Memorise this. Use the anchors E=5, J=10, O=15, T=20, Y=25 to find any letter fast.
Opposite (reverse) position: Position from the end = 27 − position from the start; For example, D is 4th from the start, so it is 23rd from the end (W). Use it when the code is based on reverse alphabet order.
Sum of letter values: Code = sum of the positions of all letters; For example, CAT = 3 + 1 + 20 = 24. Check whether the question uses sum, product or side-by-side digits.
Shifted position code: Code of letter = position ± k; First find k from the given example, then apply the same k to every letter. Check it on all examples.
Digit placement: Side-by-side coding: CAB → 3, 1, 2 → 312; Do not add the digits unless the question says the code is a sum.

How to solve Coding-Decoding: Number and Substitution Coding questions

Use this method for any number or substitution coding question. Do not guess the rule from one example.

1Read the question and note what is asked: encode (word to code), decode (code to word) or find a meaning.
2Write the given examples in two rows: original on top, code below. Keep them aligned item by item.
3Guess the type of rule: alphabet position, sum or product, a fixed shift, or a pure swap of names.
4Test the rule on every given example, not only the first. If any example fails, change the rule.
5For substitution questions, write the new names as a chain, for example green → yellow. Then follow the chain carefully.
6Apply the confirmed rule to the question item by item. Write out each letter's value on paper.
7Check the answer against the four options. Only one should match exactly, including digit order.
8If two options look close, re-check the position of each letter and the order of digits.

Quickest way: Option elimination with the alphabet anchors

When to use it: Use it in the objective paper when time is short. Most coding questions can be done in under a minute.

Learn the anchors E=5, J=10, O=15, T=20, Y=25 so you never count from A.
Find the rule from the first example only to get a candidate, then test it on the second example.
Before computing the full answer, check one detail on the options, such as the first digit, the number of digits or the last digit. Cross out options that fail.
Only compute fully if two or more options survive.
For substitution questions, ignore the real meaning of words. Follow the chain exactly as given.
If you cannot see the rule within about a minute, skip the question. A wrong answer costs 0.25 marks.

Common mistakes in Coding-Decoding: Number and Substitution Coding

Using the wrong alphabet position, such as taking M as 12 or counting from the wrong end.
Students count in a hurry and slip by one.
Fix: Use the anchors E=5, J=10, O=15, T=20, Y=25 and count from the nearest one.
Confusing the sum of values with side-by-side digits.
Students assume one type of number coding for all questions.
Fix: Check the given example. If CAB is 312, the digits are side by side. If CAT is 24, the values are added.
Testing the rule on only one example.
The first rule that fits feels right, so students stop checking.
Fix: Test every given example. A rule is accepted only when it works for all of them.
Answering a substitution question with the real meaning instead of the coded name.
Everyday knowledge overrides the question's rules.
Fix: Find the real item first, then look up what it is called in the code. Answer with that name.
Mixing up the direction, encoding when the question asks for decoding.
Students skim the question and see only the examples.
Fix: Underline what is asked. Note whether you go from word to code or from code to word.
Writing the digits of a multi-digit letter value in the wrong order, such as 15 as 51.
Students rush when joining digits for letters like O, P or Z.
Fix: Write each letter's value separately, then join them in the same order as the letters.

Worked examples

Example 1

In a certain code, CAB is written as 312. How is FACE written in that code? (A) 6135 (B) 6153 (C) 5136 (D) 6315

Show the solution

Check the rule: C = 3, A = 1, B = 2. Written side by side, this gives 312, which matches.
So each letter is replaced by its alphabet position, and the digits are written in the same order.
F = 6, A = 1, C = 3, E = 5.
Join them in order: 6, 1, 3, 5 → 6135.
Check the options: only (A) is 6135.

Answer: (A) 6135

Example 2

In a certain code, 'red' is called 'blue', 'blue' is called 'green', 'green' is called 'yellow' and 'yellow' is called 'white'. What is the colour of grass called in this code? (A) Blue (B) Green (C) Yellow (D) White

Show the solution

Write the chain: red → blue, blue → green, green → yellow, yellow → white.
Grass is really green in colour.
Look up green in the chain: green is called 'yellow'.
Do not go further along the chain. Each colour is replaced only once.
So the colour of grass is called 'yellow'.

Answer: (C) Yellow

Example 3

In a certain code, CAT is written as 24. How is DOG written in that code? (A) 25 (B) 26 (C) 27 (D) 28

Show the solution

Test the rule: C = 3, A = 1, T = 20. Sum = 3 + 1 + 20 = 24, which matches the code.
So the code is the sum of the alphabet positions of the letters.
D = 4, O = 15, G = 7.
Sum = 4 + 15 + 7 = 26.
Check the options: 26 is option (B).

Answer: (B) 26

Exam tips

Spend the first few seconds finding the rule type: position, sum, shift or swap. Do not start calculating before this.
Write the alphabet anchors (E, J, O, T, Y) at the top of your rough sheet and use them throughout the question.
Read substitution questions twice. They are easy marks only if you follow the chain exactly as given.
Options often differ in digit order or by one in the sum. Re-check your arithmetic before marking.
Because of the 0.25 negative marking, skip a question if you cannot find a rule that works on all the given examples.

Practice questions from Number Series, Coding-Decoding and Odd Man Out

Coding-Decoding: Number and Substitution Coding: frequently asked questions

What is the difference between letter coding and number coding?

In letter coding, letters are changed into other letters, usually by a shift or by reversing the alphabet. In number coding, letters or words are changed into numbers, often by alphabet position or by adding or multiplying those positions. The method is the same: find the rule from the examples and apply it.

How do I solve number coding questions quickly?

Memorise the alphabet positions with the anchors E=5, J=10, O=15, T=20 and Y=25. Find the rule from the first example and test it on the second. Then check simple features of the options, such as the number of digits, before doing full calculations.

What is substitution coding?

In substitution coding, an item is given a new name by the question. For example, 'green' may be called 'yellow'. You must answer using the new names and ignore the real ones. Write the chain of replacements and use each one only once.

Is there negative marking for coding-decoding questions?

Yes. Quantitative Aptitude is an objective paper with negative marking of 0.25 marks for each wrong answer. If you cannot find a rule that fits all the examples, it is better to skip the question than to guess.