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Jan13

Strategy for Mental Calculation: Add and Subtract Numbers

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In this post, we are going to learn a strategy for mental calculation 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:

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.

If you want to practice these strategies for mental calculation and much more elementary math, register at Smartick and try it for free!

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