In this post, we are going to learn a mental math trick for adding and subtracting various numbers. It’s all about grouping the numbers that can give us exactly ten. In order to do so, we really need to pay attention to the numbers.
Mental Math: Addition
In order to add, we need to see if the problem we’re working on has any of these pairs:
1 + 9 = 10
2 + 8 = 10
3 + 7 = 10
4 + 6 = 10
5 + 5 = 10
6 + 4 = 10
7 + 3 = 10
8 + 2 = 10
9 + 1 = 10
For example:
5 + 6 + 5 + 1
If we look at the problem, we see 5 + 5 = 10. After, we only have to add the rest of the numbers: 10 + 6 + 1 = 17
Let’s look at another example:
12 + 14 + 18
In this problem, we first add 12 + 18 = 30, and after 30 + 14 = 44
Mental Math: Subtraction
In order to subtract, we have to look for the numbers that give us exactly 10 when we subtract them.
For example:
16 – 4 – 6
First we subtract 16 – 6 = 10, and after 10 – 4 = 6
Let’s look at another example:
38 + 15 – 18
We’ll first subtract 38 – 18 = 20, and then add 20 + 15 = 35
By solving this way, we can add and subtract various numbers more easily.
Learn More:
- The Associative Property in Addition and Multiplication
- Mental Calculation Strategies: Add and Subtract Numbers
- Mental Calculation: Horizontal Addition and Subtraction
- Strategies for Solving Incomplete Horizontal Additions and Subtractions
- Incomplete Horizontal Addition and Subtraction to Develop Mathematical Flexibility
A multidisciplinary and multicultural team made up of mathematicians, teachers, professors and other education professionals!
They strive to create the best math content possible.