Multiply 12.21 by 100: Decimal Number Calculation

Q: Why do I move the decimal point instead of doing regular multiplication?

Moving the decimal point is a shortcut that works specifically with powers of 10 (10, 100, 1000, etc.). It's much faster than traditional multiplication and always gives the same result!

Q: What if there aren't enough digits to move the decimal point?

Add zeros to the right as needed! For example, $ 1.2 × 1000 $ becomes 1200 by adding two zeros after moving the decimal point three places.

Q: How do I remember which direction to move the decimal?

Think of it this way: multiplication makes numbers bigger, so the decimal point moves right. Division makes numbers smaller, so it moves left.

Decimal Multiplication with Powers of Ten

$\text{12}.21\times100=$

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations

00:00 Solve

00:03 According to the amount of zeros, move the decimal point

00:09 Move the decimal point according to the number of zeros

00:17 And this is the solution to the question

Step-by-step written solution

Follow each step carefully to understand the complete solution

Understand the problem

$\text{12}.21\times100=$

Step-by-step solution

To solve this problem, we'll follow these steps:

Step 1: Identify the initial position of the decimal point in $12.21$ , which is between the digits $2$ and $1$ .
Step 2: Shift the decimal point two places to the right. This movement changes $12.21$ to $1221$ .
Step 3: Confirm the resulting number, $1221$ .

By multiplying $12.21$ by $100$ , we shift the decimal point two spaces to the right, resulting in the number $1221$ .

Final Answer

$1221$

Key Points to Remember

Essential concepts to master this topic

Rule: Multiplying by 100 moves decimal point two places right
Technique: Count zeros in multiplier: 100 has 2 zeros = move 2 places
Check: Verify $1221 ÷ 100 = 12.21$ gives original number ✓

Common Mistakes

Avoid these frequent errors

Moving decimal point the wrong direction
Don't move the decimal point left when multiplying by 100 = 1.221 instead of 1221! This treats multiplication like division. Always move the decimal point right when multiplying by powers of 10.

Practice Quiz

Test your knowledge with interactive questions

$\text{0}.07\times10=$

FAQ

Everything you need to know about this question

Why do I move the decimal point instead of doing regular multiplication?

Moving the decimal point is a shortcut that works specifically with powers of 10 (10, 100, 1000, etc.). It's much faster than traditional multiplication and always gives the same result!

What if there aren't enough digits to move the decimal point?

Add zeros to the right as needed! For example, $1.2 × 1000$ becomes 1200 by adding two zeros after moving the decimal point three places.

How do I remember which direction to move the decimal?

Think of it this way: multiplication makes numbers bigger, so the decimal point moves right. Division makes numbers smaller, so it moves left.

What if my answer doesn't have a decimal point showing?

That's correct! When you move the decimal point all the way to the right end, it becomes a whole number. The decimal point is still there, just invisible at the end.

Does this trick work with other numbers besides powers of 10?

No, this shortcut only works with 10, 100, 1000, etc. For other numbers like 25 or 37, you need to use regular multiplication methods.

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