21 in Roman numerals

 

21 in Roman numerals is written as XXI. Roman numerals are an ancient number system that originated in Rome and use letters from the Latin alphabet to represent numbers instead of the digits we use today. Each symbol has a fixed value, such as I = 1, V = 5, and X = 10, and larger numbers are formed by adding or subtracting these values according to certain rules.

In this lesson, we will learn how to write 21 in Roman numerals, explore the basic rules behind Roman numeral writing, and solve step-by-step examples for better understanding.

Table of Contents

• How to Write 21 in Roman Numerals?
• Basic Rules to Write Roman Numerals
• Roman Numerals Related to the Number 21
• Solved Examples on Roman Numerals
• Conclusion
• FAQs on 21 in Roman Numerals

 

How to Write 21 in Roman Numerals?

Writing 21 in Roman numerals requires us to understand how numbers are formed using the basic Roman numeral symbols. The primary symbols we need are:

I = 1

V = 5

X = 10

L = 50

C = 100

D = 500

M = 1000

Now, to form the number 21, 

10 is represented as X

20 is represented as XX (10 + 10)

1 is represented as I

Therefore, 21 = XX + I = XXI

So, 21 in Roman numerals is written as XXI.

This follows the basic Roman numeral rule that when a smaller value symbol comes after a larger one, the values are added. In this case, XX (20) is followed by I (1), giving us 21.

Basic Rules to Write Roman Numerals

• A symbol is repeated when added:

Example: II = 2, XX = 20, CC = 200

• A symbol can be repeated a maximum of 3 times:

Example: XXX = 30, CCC = 300

• Symbols V (5), L (50), and D (500) are never repeated.

• When a smaller symbol comes after a larger one, we add:

Example: VI = 5 + 1 = 6

• When a smaller symbol comes before a larger one, we subtract

Example: XC = 100 – 10 = 90

• Symbols V, L, and D are never subtracted.

• I can only be subtracted from V and X.

• X can only be subtracted from L and C.

 

Roman Numerals Related to the Number 21

The following are some numbers near 21 and their Roman numeral forms:

20 in Roman Numerals = XX

21 in Roman Numerals = XXI

22 in Roman Numerals = XXII

23 in Roman Numerals = XXIII

25 in Roman Numerals = XXV

30 in Roman Numerals = XXX

This shows how numbers are built by combining the symbols logically.

 

Solved Examples on Roman Numerals

Example 1: Express the sum of XV (15) and VI (6) in Roman numerals.

Solution:

We know:
XV = 15
VI = 6

Adding them gives:

15 + 6 = 21

Now, express 21 in Roman numerals:

21 = XXI

Therefore,

XV + VI = XXI

Example 2: What is to be added to XX (20) to obtain XXI?

Solution:

We know:
XXI = 21
XX = 20

To find the number to add, subtract 20 from 21:
21 – 20 = 1

Now, express 1 in Roman numerals:
1 = I

Therefore,
XX + I = XXI

Example 3: Express the difference between XXX (30) and IX (9) in Roman numerals.

Solution:

We know:

XXX = 30

IX = 9

Subtract:

30 – 9 = 21

Now, write 21 in Roman numerals:

21 = XXI

Therefore,

XXX – IX = XXI 

Example 4: Multiply VII (7) × III (3) in Roman numerals.

Solution:

We know:

VII = 7

III = 3

Multiply:

7 × 3 = 21

Now, write 21 in Roman numerals:

21 = XXI

Therefore,

VII × III = XXI

Conclusion

The Roman numeral for 21 is XXI. It is formed by combining XX (20) and I (1). Roman numerals have certain rules of addition and subtraction, which are quite simple once the basic values are learned. With practice, such numbers as 21 (XXI), 25 (XXV), and 30 (XXX) can be quickly written and read.

 

Frequently Asked Questions on 21 in Roman Numerals

1. What is 21 in Roman numerals?

Answer: 21 in Roman numerals is represented as XXI.

2. How is 21 constructed in Roman numerals?

Answer: 20 is XX and 1 is I. Therefore, 21 = XX + I = XXI.

3. What is after XXI in Roman numerals?

Answer: After XXI (21), we have XXII (22).

4. Are Roman numerals applicable for huge numbers?

Answer: Yes, Roman numerals can be used to represent very huge numbers. To represent numbers above 3999, a bar is drawn over the numeral, increasing its value by 1000. 

Learn 21 in Roman Numerals and other mathematical concepts at Orchids The International School.