Solve Vertical Subtraction: 91 Minus 90 Step-by-Step

Q: Why do I get 0 in the tens place?

When subtracting 9 - 9 = 0, you get zero! This is correct - write the 0 down. Since there are no hundreds, your final answer is just 1.

Q: Do I need to borrow when subtracting 1 - 0?

No borrowing needed! Since 1 is larger than 0, you can subtract directly. Borrowing is only needed when the top digit is smaller than the bottom digit.

Q: How can I check if 1 is the right answer?

Use addition to check subtraction: $1 + 90 = 91$. Since this equals your starting number, your answer is correct!

Q: What if the numbers were switched to 90 - 91?

You'd need to learn about negative numbers or borrowing from zero. For now, make sure the larger number is on top in vertical subtraction.

Q: Why is vertical subtraction better than doing it in my head?

Vertical alignment helps you subtract the correct digits! It prevents mixing up tens and units, especially with larger numbers or when borrowing is needed.

Vertical Subtraction with Two-Digit Numbers

$\begin{aligned} &91 \\ -& \\ &90 \\ &\underline{\phantom{776}} & \\ \end{aligned}$

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations

00:00 Solve

00:03 Each time we calculate subtraction of 2 digits, and then we substitute

00:06 We'll start by subtracting ones from ones, and place in ones

00:10 We subtract tens from tens, and place in tens

00:13 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

$\begin{aligned} &91 \\ -& \\ &90 \\ &\underline{\phantom{776}} & \\ \end{aligned}$

Step-by-step solution

To solve this subtraction problem, we will perform the operation vertically, step by step:

Step 1: Write the two numbers in vertical alignment:
91
- 90
---
Step 2: Start subtracting from the rightmost column (the units column):
Subtract the units: $1 - 0 = 1$ .
Step 3: Move to the tens column:
Subtract the tens: $9 - 9 = 0$ .

So, after performing the subtraction, the result is $1$ .

Therefore, the solution to the problem is $1$ .

Final Answer

Key Points to Remember

Essential concepts to master this topic

Alignment: Place numbers with units column under units column
Technique: Start rightmost: $1 - 0 = 1$ , then tens: $9 - 9 = 0$
Check: Add your answer to the bottom number: $1 + 90 = 91$ ✓

Common Mistakes

Avoid these frequent errors

Subtracting top number from bottom number
Don't calculate 90 - 91 = wrong answer! This reverses the subtraction order and gives negative results or confusion. Always subtract the bottom number from the top number in vertical subtraction.

Practice Quiz

Test your knowledge with interactive questions

$\begin{aligned} &105 \\ -& \\ &~~~~3 \\ &\underline{\phantom{776}} & \\ \end{aligned}$

FAQ

Everything you need to know about this question

Why do I get 0 in the tens place?

When subtracting 9 - 9 = 0, you get zero! This is correct - write the 0 down. Since there are no hundreds, your final answer is just 1.

Do I need to borrow when subtracting 1 - 0?

No borrowing needed! Since 1 is larger than 0, you can subtract directly. Borrowing is only needed when the top digit is smaller than the bottom digit.

How can I check if 1 is the right answer?

Use addition to check subtraction: $1 + 90 = 91$ . Since this equals your starting number, your answer is correct!

What if the numbers were switched to 90 - 91?

You'd need to learn about negative numbers or borrowing from zero. For now, make sure the larger number is on top in vertical subtraction.

Why is vertical subtraction better than doing it in my head?

Vertical alignment helps you subtract the correct digits! It prevents mixing up tens and units, especially with larger numbers or when borrowing is needed.

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