In the unit Introduction to Decimals, we study decimal numbers with one or two decimal digits, and add and subtract them. It is important that the student grasps these simple topics well, because we are laying a groundwork towards fifth and sixth grade, where decimal operations and using decimals take "center stage."
For now, the focus is first of all understanding the fact that decimals are simply fractions with a denominator of 10 or 100. Then with that in mind (decimals are fractions), we study comparing, adding, and subtracting them.
In the addition problem 0.5 + 0.9, we get 14 tenths, which is 1.4. A common student misconception is to add 0.5 + 0.9 = 0.14. In a problem such as 0.5 + 0.11, a common student misconception is to get 0.16. Such students are thinking of the decimal parts as if they were "whole numbers." To solve 0.5 + 0.11 correctly, students can rewrite 0.5 as 0.50, and then the problem becomes 0.50 + 0.11 = 0.61.
In the lesson Using Decimal Numbers, students use decimals with some metric measuring units, including converting between units.
I wish you success in teaching math!
Maria Miller, the author