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What is the Number 21?
Grade Level:
Pre-School – Class 2
All domains without exception
Definition
What is it?
The number 21 is a natural number that comes after 20 and before 22. It is a two-digit number, where '2' is in the tens place and '1' is in the ones place. We can also think of 21 as twenty plus one (20 + 1).
Simple Example
Quick Example
Imagine your favourite cricket team scored 20 runs, and then the batsman hit one more run. Their total score would be 21 runs. This is how we use 21 in everyday life.
Worked Example
Step-by-Step
Let's say you have 10 laddoos, and your friend gives you 11 more laddoos. How many laddoos do you have in total?
Step 1: Start with the number of laddoos you have: 10.
---Step 2: Add the number of laddoos your friend gave you: 11.
---Step 3: Set up the addition: 10 + 11.
---Step 4: Add the ones digits: 0 + 1 = 1. Write '1' in the ones place of the answer.
---Step 5: Add the tens digits: 1 + 1 = 2. Write '2' in the tens place of the answer.
---Step 6: Combine the digits to get the total.
Answer: You have 21 laddoos in total.
Why It Matters
Understanding numbers like 21 is fundamental for all areas of maths, from basic counting to complex algebra. It's used by shopkeepers to calculate change, by engineers to measure distances, and even by scientists to count data. Mastering basic numbers helps you build a strong foundation for future learning and various careers.
Common Mistakes
MISTAKE: Writing '12' instead of '21' when asked to write 'twenty-one' | CORRECTION: Remember that 'twenty' means two tens (20), so the '2' comes first, followed by the '1' for 'one'.
MISTAKE: Confusing the value of the digits, thinking the '1' in '21' represents ten | CORRECTION: In 21, the '2' is in the tens place (value 20) and the '1' is in the ones place (value 1).
MISTAKE: Counting incorrectly when adding to reach 21, for example, 10 + 10 = 21 | CORRECTION: Always double-check your addition. 10 + 10 = 20, not 21. 10 + 11 = 21.
Practice Questions
Try It Yourself
QUESTION: What number comes just after 20? | ANSWER: 21
QUESTION: If you have 15 rupees and find 6 more rupees, how much money do you have in total? | ANSWER: 21 rupees
QUESTION: A bus has 10 seats on one side and 10 seats on the other side. If there is one extra seat near the driver, what is the total number of seats? | ANSWER: 21 seats
MCQ
Quick Quiz
Which of these sums equals 21?
10 + 10
15 + 5
18 + 3
20 + 2
The Correct Answer Is:
C
18 + 3 = 21. Options A, B, and D give 20, 20, and 22 respectively, which are not 21.
Real World Connection
In the Real World
In India, you might see the number 21 in many places. For example, a shopkeeper might tell you that the total bill for your groceries is 21 rupees. Or, if you are playing carrom, you might need to score 21 points to win the game.
Key Vocabulary
Key Terms
NATURAL NUMBER: A whole positive number used for counting (1, 2, 3, ...)| DIGIT: A single symbol used to write numbers (0, 1, 2, ... 9)| TENS PLACE: The position of a digit representing groups of ten| ONES PLACE: The position of a digit representing single units
What's Next
What to Learn Next
Great job understanding the number 21! Next, you should explore larger numbers and learn about their place values. This will help you count, add, and subtract even bigger numbers with confidence.
