5.5 Introduction and Key Insights
It can be simple to notate adding whole numbers, such as 
. However, while understanding the computation 
is easy, it is more complicated to notate than our first sum. The numbers four and eight each use only one digit, but the sum is two digits. In other words, adding four and eight together requires us to use a new place value: a one in a new "ten's place".
This example shows us that adding and subtracting can require using new positions to the left of the decimal point. When we add or subtract decimal expressions, we often have to change positions to the right of the decimal point as well. For example, consider this equation, which involves fractions:
When we write this equation using decimals, we have:
To the left side of the equal sign, we have two decimal expressions that each uses one digit to the right of the decimal point, but once we sum them together, we no longer need to use any digits to the right of the point. To keep track of the positions of the digits of our final answer use, we need to match the positions of each decimal expression up to make sure we line up the decimal points as well.