Lesson objective: Extend understanding that representations and drawings can be used to solve one-step situational problems involving addition and subtraction of grams and kilograms.

Students bring prior knowledge of using a number line to show methods for solving addition and subtraction of milliliters and liters from Grade 3, Unit 6, 3.MD.A.2. This prior knowledge is extended to grams and kilograms as students solve one-step situational problems involving addiiton and subtraction. A conceptual challenge students may encounter is understanding the concept of grams and kilograms because their prior experience is only with pounds and ounces. Another conceptual challenge students may encounter is adding three-digit numbers using multiple strategies based on place value.

The concept is developed through work with a number line, which encourages students to show methods of adding and subtracting that are not the traditional algorithm. The students focus on the place value of the numbers.

This work helps students deepen their understanding of operations because the students use pictures and representations to understand the problem and meaning of the question in order to determine the correct operation to use. Also, students continue to deepen their understanding that addition and subtraction are inverse operations. Students deepen their understanding of numbers as they decompose numbers into smaller numbers. Also, students deepen their understanding that 1,000 grams = 1 kilogram.

Students engage in Mathematical Practice 1 (make sense of problems and persevere in solving them) as they use pictures and representations to solve problems about grams and kilograms using addition and subtraction. Students engage in Mathematical Practice 4 (model with mathematics) as they use number equations to show how they used the number line to solve the problem.

**Key vocabulary:**

- gram
- kilogram
- mass
- matter
- number line
- pan balance scale

**Special materials needed:**

- kitchen gram scale
- number line for demonstration
- objects to find mass on a gram scale
- pan balance scale, gram weights