Use Calculation Strategies: Adding and Subtracting Near Multiples of 10, by Adjustment

Sometimes, to do addition and subtraction questions we can use the method of adjustment. 

This method works for decimals that are very close to whole numbers, or numbers that are very close to tens or hundreds.

Example 1 => 454 + 198 = ?
198 is very close to 200. 198 = 200 – 2
It is easier to add 200 to 454 than to add 198.
454 + 200 = 654
However, this answer is 2 more than we need!
So, the adjustment is => 654 – 2 = 652

Example 2 => 1365 – 59 = ?
59 is almost 60! 59 = 60 – 1
It is easier to subtract 60 than to subtract 59.
1365 – 60 = 1305
We have taken 1 too many away. We only want to subtract 59.
Let’s adjust => 1305 + 1 = 1306

Example 3 => 11.1 + 35.6 = ?
This sum can be done by adding 11, then, adding 0.1 to adjust.
35.6 + 11 = 46.6
46.6 + 0.1 = 46.7

Example 4 => 123 – 17.2 = ?
17.2 is close to 17. We can subtract 17, then, subtract 0.2 to adjust.
123 – 17 = 106
Adjustment => 106 – 0.2 = 105.8

Now try a few practice questions!