In previous lessons, students used strategies based on place value and properties of operations to add decimal numbers including the standard algorithm. The purpose of this lesson is to highlight the importance of adding the same place values when making vertical calculations with decimals. This issue does not arise when adding whole numbers as the numbers are always aligned to the right, starting with ones, then tens, and so on. For decimal numbers it is more complicated because the last digits in two decimals, such as 5.18 and 16.7, may not have the same place value. Rather than aligning these numbers to the right as shown, they need to be aligned by place value.

After examining this alignment error, students practice adding decimals taking care to add the correct place values.

Reflect on a time your thinking changed about something in class recently. How will you alter your teaching practice to incorporate your new understanding?