In this post, we are going to learn mental calculation strategies, which we can use when one of the two numbers is near a multiple of ten.
We round the number up to that nearest multiple of ten, and then we add or subtract the digits that are left.
We’re going to take a look at this in parts.
Index
Addition of two numbers, one of which ends in 8 or 9
We’re going to add 26 + 9.
Since 9 is close to 10, we can replace the 9 with 10 – 1. This way, we are left with 26 + 10 – 1.
Now we add 26 + 10 = 36, and we are left with 1 to subtract: 36 – 1 = 35.
Therefore, 26 + 9 = 35.
Addition of two numbers, one of which ends in 1 or 2
We are going to add 24 + 11.
Since 11 is close to 10, we can replace the 11 with 10 + 1. And we are left with 24 + 10 + 1.
Now we add 24 + 10 = 34. Lastly, we add the 1 that was leftover: 34 + 1 = 35.
Therefore, 24 + 11 = 35.
Subtraction of two numbers with the subtrahend ending in 8 or 9
We are going to subtract 54 – 28.
Since 28 is close to 30, we can write the 28 as 30 – 2, and we are left with 54 – (30 – 2).
Given there is a negative sign in front of the parentheses, the operation would be 54 – 30 + 2.
No we subtract 54 – 30 = 24. Lastly, we add 24 + 2 = 26.
Therefore, 54 – 28 = 26.
Subtraction of two numbers, with the minuend ending in 8 or 9
We are going to subtract 29 – 14.
Since 29 is close to 30, we can write 29 as 30 – 1. This way we are left with 30 – 1 – 14.
Now we are going to subtract 30 – 14 = 16. Lastly, we subtract the 1 that was leftover 16 – 1 = 15.
Therefore, 29 – 14 = 15.
Subtraction of two numbers, with the minuend ending in 1 or 2
We are going to subtract 32 – 24.
Since 32 is close to 30, we can write it as 30 + 2. Which leaves us with 30 + 2 – 24.
Now we subtract 30 – 24 = 6. Lastly, we add the 2 that was leftover 6 + 2 = 8.
Therefore, 32 – 24 = 8.
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Learn More:
- Mental Calculation: Horizontal Addition and Subtraction
- How to Solve a Subtraction Problem with Regrouping
- Learn How to Perform Combined Operations
- Strategies for Solving Incomplete Horizontal Additions and Subtractions
- Combined Operations: How to Solve These Types of Problems
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