Learn all about subtracting two-digit numbers and subtracting three-digit numbers in this free lesson, which includes practice problems.
In Introduction to Subtraction, we learned that counting and using visuals can be useful for solving basic subtraction problems. For instance, say you have 9 apples and you use 6 to make a pie. To find out how many apples are left, you could represent the situation like this:
It's easy to count and see that 3 apples are left.
What if you need to solve a subtraction problem that starts with a large number? For instance, let's say instead of making an apple pie, you want to pick apples from an apple tree. The tree has 30 apples and you pick 21. We could write this as 30 - 21.
You might see why counting to solve this problem isn't a good idea. When you have a subtraction problem that starts with a large number, it could take a long time to set up the problem. Imagine the time it would take to count out 30 objects and then take away 21! Also, it would be easy to lose track as you counted. You could end up with the wrong answer.
For this reason, when people solve a subtraction problem with large numbers, they set up the problem in a way that makes it easy to solve one step at a time. Let's see how this works with another problem: 79 - 13.
In the last lesson, we learned how to write expressions. However, subtracting with larger numbers is easier when the expressions are written in a different way.
Instead of writing the numbers side by side…
Place the numbers so they are stacked — one number on top and one number on the bottom.
With a stacked subtraction expression, the larger number is always written on top. Here, that number is 79.
Write the amount being subtracted underneath the top number. That's 13.
Put the minus sign to the left of the numbers.
Instead of an equals sign, put a line underneath the bottom number.
When you stack a subtraction expression, make sure the numbers are lined up correctly. They are always lined up on the right. Here, we lined up 9 and 3.
Here's another problem, 576 - 2. With this problem, see how we lined up the numbers to the right?
No matter how many digits are in the numbers, always line up the numbers to the right.
We can see that 79 - 13 and mean the same thing — they're just written differently.