Adding and Subtracting Decimals

 

Adding and Subtracting Decimals

Adding Decimals

To add decimals, follow these steps:

  • Write down the numbers, one under the other, with the decimal points lined up
  • Put in zeros so the numbers have the same length (see below for why that is OK)
  • Then add using column addition, remembering to put the decimal point in the answer

Example: Add 1.45 to 1.3

Line the decimals up:1.45
+1.3
"Pad" with zeros:1.45
+1.30
Add:1.45
+1.30
2.75

Example: Add 3.25, 0.07 and 5

Line the decimals up:3.25
0.07
+5.
"Pad" with zeros:3.25
0.07
+5.00
Add:3.25
0.07
+5.00
8.32
That's all there is to it - just remember to line up the decimals, then add normally.

Subtracting Decimals

To subtract, follow the same method: line up the decimals, then subtract.

Example: What is 7.368 − 1.15 ?
Line the decimals up:7.368
−1.15
"Pad" with zeros:7.368
−1.150
Subtract:7.368
−1.150
6.218

To check we can add the answer to the number subtracted:

Example (continued): Check by adding 6.218 to 1.15

Line the decimals up:6.218
+1.15
"Pad" with zeros:6.218
+1.150
Add:6.218
+1.150
7.368
It matches the number we started with, so it checks out.
Adding Zeros

Why can we add zeros?
A zero is really saying "there is no value at this decimal place".

In a number like 10, the zero is saying "no ones"
In a number like 2.50 the zero is saying "no hundredths"
So it is safe to take a number like 2.5 and make it 2.50 or 2.500 etc.

But DON'T take 2.5 and make it 20.5, that changes the number.

Here is a song we listened to to help us remember what to do:

Line Em Up Song

Here are some videos to watch:

StudyJams Addition and Subtraction of Decimals

StudyJams Making Change

Here are some games to play to practice this skill:

Adding Decimals Soccer Game

Subtracting Decimals Soccer Game

Hungry Puppies Adding Decimals

Decimal Subtraction

Decimal Addition


Math at the Mall

Please let me know if you have any questions!

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