112 LearnersLast updated on May 26th, 2025
21 in Roman Numerals
Roman numerals are a way of expressing numbers using symbols. The symbols I, V, X, L, C, D, and M are used in this system. Roman Numerals are often used in royal titles, book names, sequences, and more. In this article, we will be discussing Roman Numerals, their rules, and examples.
What is 21 in Roman Numerals?
You might have noticed Roman Numerals in royal titles such as Henry I, Henry II, and so on. These symbols represent numbers in the Roman numeral system. Earlier civilizations used fingers, sticks, bones, etc., for counting. As life became more complex, a standardized form like Roman Numerals was adopted. Ancient Romans used this system to count with symbols like I (1), V (5), X (10), L (50), C (100), D (500), and M (1000).
In Roman Numerals, we use XXI to represent 21, where X is 10, another X is 10, and I is 1. Let’s learn more about Roman numerals and how we write them.
Basic Rules for 21 in Roman Numerals
There are basic rules to write numbers in Roman Numerals. Let's discuss some of these rules:
Rule 1: Addition Method
The addition method is used when a smaller number is placed after a larger number. For example, XXI → X + X + I → 10 + 10 + 1 = 21.
Rule 2: Repetition Method
Certain Roman Numerals can be repeated up to three times. For example, XXX = 30 (X is repeated three times).
Rule 3: Subtraction Method
If a smaller number precedes a larger number, it is subtracted. For example, IX → X - I → 10 - 1 = 9.
Rule 4: Limitation Rule
Symbols can’t be repeated more than three times. Symbols like V, L, and D cannot be repeated. For example, we write 8 as VIII, not IIIIIIII.
How to Write 21 in Roman Numerals
Let’s now learn how to write 21 using Roman Numerals. We can do this using the Expansion Method and the Grouping Method.
21 in Roman Numerals by Expansion Method
In the expansion method, the number is broken down based on its place value.
To write 21 in Roman Numerals, follow these steps:
1. Break down the number based on place value: 20 + 1.
2. Convert to Roman Numerals:
20 in Roman Numerals — XX.
1 in Roman Numerals — I.
3. Combine the Roman Numerals: Therefore, 21 is XX (20) + I (1) = XXI.
21 in Roman Numerals by Grouping Method
When writing a large number in Roman Numerals, group the number:
1. Group 21 as 20 + 1.
2. Convert the groups:
20 in Roman Numerals — XX.
1 in Roman Numerals — I.
3. Combine them: 21 is written as XXI.
Numbers Related to 21 in Roman Numerals
Roman Numerals follow a pattern. Here are some numbers related to the numeral 21:
XX = 20
XXI = 21
XXII = 22
XXIII = 23
XXIV = 24
XXV = 25
Common Mistakes and How to Avoid Them in 21 Roman Numerals
Students often make mistakes when writing numbers in Roman Numerals. Here are some common mistakes:
Mistake 1
Errors in Repetition
The repetition rule restricts symbols to three repetitions. Symbols like V, L, and D cannot be repeated.
Mistake 2
Incorrect Subtraction
Errors occur when subtracting the wrong numerals, such as writing IL for 49 instead of XLIX.
Mistake 3
Incorrect Grouping
When grouping numbers, ensure correct representation. For example, 6 is VI, not IIIIII.
Mistake 4
Confusing Arabic and Roman Numerals
Students may confuse Arabic and Roman systems. Understanding the basic concepts helps avoid mistakes.
Mistake 5
Confusing Subtraction and Addition
Students might confuse when to add or subtract. For example, IV is 4 because I precedes V, while XV is 15 because X precedes V.
21 in Roman Numerals Examples
Problem 1
A historian discovers XXI ancient coins and wants to divide them equally among VII friends. How many coins will each friend receive?
Each friend will receive III coins.
Explanation
To determine the number of coins each friend gets, divide the total number of coins by the number of friends.
XXI = 21
VII = 7
Therefore, XXI / VII = 21 / 7 = 3.
3 in Roman numerals is III.
Problem 2
If you add the Roman numerals for XXI and XIV, what is the result?
The sum of XXI and XIV is XXXV.
Explanation
To find the sum, add the values of the Roman numerals.
XXI = 21
XIV = 14
21 + 14 = 35
35 in Roman numerals is XXXV.
Problem 3
A collector has a total of LXXVII ancient Roman artifacts. If XXI artifacts are stolen, how many are left?
LVI artifacts are left.
Explanation
Subtract the stolen artifacts from the total number.
LXXVII = 77
XXI = 21
77 - 21 = 56
56 in Roman numerals is LVI.
Problem 4
Convert the product of VII and III into Roman numerals.
The product of VII and III is XXI.
Explanation
Multiply the given numbers:
VII = 7
III = 3
7 × 3 = 21
21 in Roman numerals is XXI.
Problem 5
Express the sum of 10 + 11 using Roman numerals in the expansion method.
The Roman numeral for 10 + 11 is XXI.
Explanation
First, convert the numbers 10 and 11 into Roman numerals.
10 - X
11 - XI
Add the numbers: 10 + 11 = 21
Using the expansion method: 20 + 1 = XXI
FAQs on 21 in Roman Numerals
1.What is 19 in Roman numerals?
2.How to write 21 in Roman numerals?
3.What is 26 in Roman Numerals?
4.Is XXI a prime number?
5.What are the multiples of 21?
Important Glossaries for 21 in Roman Numerals
- Addition Rule: Used when a larger number is followed by a smaller numeral, resulting in their addition. For example, XXI = X + X + I = 21.
- Grouping Method: Numbers are grouped based on place value, then converted to Roman numerals. For example, 21 = 20 + 1 = XXI.
- Repetition Rule: Certain symbols (I, X, C, M) can be repeated up to three times. For example, XXX = 30.
- Subtraction Method: Used when a smaller numeral precedes a larger one, indicating subtraction. For example, IV = 4.
- Limitation Rule: Limits the repetition of symbols and states some cannot be repeated. For example, V cannot be repeated.
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