Adding and Subtracting Decimals
Adding Decimals
To add decimals, follow these steps:
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 are some videos to watch:
Here are some games to play to practice this skill:
Please let me know if you have any questions!
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