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Adding and subtracting with decimals (Algorithm and Model)

To add decimals using the standard algorithm, follow these steps:

1).  Write down the numbers with the place values and decimals lined up

2).  Put in zeros so the numbers have the same amout of place values.

3).  Then add.  Make sure you bring the decimal point straight down in the answer.

Example: Add 4.336 to 3.1

Line the decimals up:     4.336
    + 3.1
       
"Pad" with zeros:     4.336
    + 3.100
       
Add:     4.336
    + 3.100
       
      7.436

Example: Add 5.23, 0.325 and 8

Line the decimals up:     5.23
      0.325
    + 8.
       
"Pad" with zeros:     5.230
      0.325
    + 8.000
       
Add:     5.230
      0.325
    + 8.000
       
      13.555

That's all there is to it - just remember to line up the decimals, then add normally.

Subtracting

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

Example: What is 7.368 − 1.15 ?

Line the decimals up:     14.578
    12.45
       
add zeros:     14.578
    12.450
       
Subtract:     14.578
    12.450
       
      6.128

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"
  • It is safe to take a number like 2.5 and make it 2.50 or 2.500, but you cannot take 2.5 and make it 20.5 because they are not the same values. 

Watch this LEARNZILLION video on how to add using the models.

Subtracting using decimals models.  Watch this video to understand how to subtract using decimal models.

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