G S Rehill's Interactive Maths Software Series - "Building a Strong Foundation in Mathematics" from mathsteacher.com.au.

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Year 8 Interactive Maths - Second Edition


Addition and Subtraction of Decimals

Knowing how to add and subtract decimals is important in life.  For example, when you handle money at a shop, the bank or post office, you should use your knowledge of decimals to calculate the money and any change exchanged.


Addition of Decimals

To add decimal numbers, insert zeros in empty decimal place values so that all of the numbers have the same number of decimal places and write the numbers such that their decimal points are below one another.


Example 7

Calculate 5.84 + 8 + 12.79.

Solution:


Note:

The decimal point in the answer is lined up with the decimal points in the numbers set out vertically.


Subtraction of Decimals

To subtract a small decimal number from a larger decimal number, write them down with the larger one on top and the decimal points underneath one another.  Then calculate the subtraction as you would for whole numbers and line up the decimal point in the answer.


Example 8

Rewrite 3.67 1.83 in columns and then calculate.

Solution:


Example 9

Rewrite 83.47 57.684 in columns and then calculate.

Solution:


Note:

To add (or subtract) decimals, always fill empty place values with zeros so that all of the numbers have the same number of decimal places.


Example 10

Rewrite 24 8.327 in columns and then calculate.

Solution:


Note:

24 = 24.000


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