This lesson strengthens students’ ability to add and subtract decimals, enabling them to work toward fluency. Students encounter longer decimals (beyond thousandths), find unknown addends, and work with decimals in the context of situations. They decide which operation (addition or subtraction) to perform and which strategy to use when finding sums and differences.

The lesson also reinforces the idea that we can express a decimal in different but equivalent ways, and that writing additional zeros after the last digit to the right of the decimal point in a number does not change its value. Students use this understanding to practice subtracting numbers with more decimal places from those with fewer decimal places (such as, ).

To solve these problems, students must lean heavily on their understanding of base-ten numbers and make use of structure (MP7). Given problems such as and , they need to think carefully about the meaning of each place value, the meaning of addition and subtraction, and potential paths toward the solution. In thinking about what symbols represent and how to represent base-ten values, students reason abstractly and quantitatively (MP2).