Decimals: Addition & Subtraction

I. Standard Algorithm

To add or subtract numbers with decimals:
1) Place the numbers in vertical form with the decimal points aligned.
2) Add or subtract each column of numbers the same as you would add or subtract whole numbers.
3) Place the decimal point in the answer directly below the decimal point in the terms.

Example: Add 8.53 + 15.2

Write the numbers in vertical form with the decimals aligned. You can add zeros to fill empty spaces.

     8.53
+ 15.20

Add as you would normally add whole numbers and place the decimal in the answer directly below the decimal point in the terms.

     8.53
+ 15.20
   23.73

Example: 95.2 - 23.15

Write the numbers in vertical form with the decimals aligned. You can add zeros to fill empty spaces.

   95.20
-  23.15

Subtract as you would normally subtract whole numbers and place the decimal in the answer directly below the decimal point in the terms.

        1 10
   95.20
-  23.15
   72.05

II. Conceptual Understanding

Base Ten Blocks

1.21 + 0.2

Here the problem is represented using base-ten blocks with the large square representing one whole, the rectangular rod representing tenths and the smallest blocks representing hundredths. 

This representation allows you to see the problem in various ways. For instance, 1.21 + 0.2 could also be thought of as 121 hundredths + 20 hundredths = 141 hundredths. 

Practice:

Khan: Adding decimals (tenths)
Khan: Adding decimals (hundredths)
Khan: Subtracting decimals (tenths)

Khan: Subtracting decimals (hundredths)
K5 Learning: Decimal Addition & Subtraction for Grade 5

Reference:
Math.com: Adding & Subtracting Decimals