Lesson objective: Apply understanding of place value to determine what strategy can be used most efficiently to solve subtraction problems.

This lesson provides an opportunity for students to apply their knowledge and understanding of place value to a mathematical situation. Students are asked to evaluate subtration problems to determine if decomposition is an efficient strategy choice.

Key Concept students will use:

- We can subtract numbers within 100 using our knowledge of place value.

Skills students will use:

- decomposing tens and ones (Grade 1, Unit 9)
- subtracting tens and ones (Grade 1, Unit 14)
- applying strategies to subtract two-digit numbers

Students engage in Mathematical Practice 7 as they look for and make use of structure, which is central to the work in this task. Students should identify the structure of place value in order to add or subtract within 100 as they consider several subtraction equations and determine what problems could best be solved using decomposition.

**Key vocabulary: **

- decomposing
- difference
- ones
- place value
- tens

**Special materials needed:**

- place value tools for students who need concrete representations