Lesson objective: Understand that adding and subtracting decimals uses the same methods as adding and subtracting whole numbers.

Students bring prior knowledge of adding and subtracting whole numbers from 4.NBT.4B.4. This prior knowledge is extended to decimals as students add and subtract money. A conceptual challenge students may encounter is using zero as a place holder in subtraction if numbers do not have the same number of digits to the left of the decimal.

The concept is developed through work with number lines, which make explicit the connection between whole numbers and decimal numbers.

This work helps students deepen their understanding of operations because they compose and decompose units to add and subtract decimals just as they do with whole numbers.

Students engage in Mathematical Practice 4 (Modeling with mathematics) as they solve the real-world problem of adding and subtracting decimals in cash transactions. Students also use mathematical representations of a number line to model the increase when decimals are added.

Students engage in Mathematical Practice 7 (look for and make use of structure) as they deepen and broaden their understanding of place value and the base 10 number system beyond whole numbers to decimals by decomposing the space between whole numbers on a number line to the 100ths.

**Key vocabulary:**

- compose
- decimal point
- decompose
- unit of value

**Special materials needed:**

- number lines
- money