Contributor: Erika Wargo. Lesson ID: 12390
Algorithm is not a new kind of music; it's a series of steps used to complete a task. In this case, the task is subtraction, and the algorithm is easy and fun, with games, practice and a simple video!
It was a nice day, and they were hungry for blueberry pie. So, they went to the farm, where Tina collected 981 blueberries and Pablo collected 456.
Subtraction is separating one group into two groups or taking one number away from another.
When subtracting one number from another number, the answer is called the difference. A common method used to subtract numbers is called the standard subtraction algorithm.
Whoa!
An algorithm is a set of steps that are used to complete a task.
In this lesson, the steps are used to perform subtraction.
The standard algorithm for subtraction has three steps:
The greater number is written, or stacked, above the number with a smaller value.
It is also helpful to use grid paper to help you line up the digits in each number.
Example 1
721  358
Step 1
Line up the numbers by place value.
Since both numbers have three digits in them, each digit in the first number should be lined up with another digit in the second number:
7 
2 
1 

 
3 
5 
8 

Step 2
Begin subtracting in the ones place.
No, so you need to borrow and regroup.
Step 3
Borrow and regroup.
The 2 tens and 1 ones becomes 1 ten and 11 ones. In the ones place, 11  8 = 3.
Move to the tens place and continue following the subtraction steps:
1 

7 
2 
^{1}1 

 
3 
5 
8 

3 
No, so you need to borrow and regroup.
Borrow and regroup from the hundreds. The 7 hundreds and 1 tens becomes 6 hundreds and 11 tens.
6 
^{1}1 

7 
2 
^{1}1 

 
3 
5 
8 

3 
6 
3 
Subtract the tens place: 11  5 = 6.
Subtract the hundreds place: 6  3 = 3.
363 is the difference.
A subtraction answer can be checked by adding the difference to the number subtracted. The sum of the addition should equal the starting number.
To check: 363 + 358 = 721, which is the number we started with:
^{1} 
^{1} 

3 
6 
3 

+ 
3 
5 
8 

7 
2 
1 
Example 2
At the beginning of the lesson, you were asked the following question:
Tina collected 981 blueberries and Pablo collected 456.
Using the standard algorithm, line up the digits and subtract, starting with the ones place:
7 

9 
8 
^{1}1 

 
4 
5 
6 

5 
2 
5 
Tina collected 525 more blueberries than Pablo.
Now, watch a few more examples in Math Antics  MultiDigit Subtraction (below). Listen for the answers to these questions and write the answers on a piece of paper, if necessary:
In the Got It? section, you will practice subtracting numbers!