Learning Intention: Students will be able to subtract two-digit numbers within 20.

Lesson Outline

I. Introduction (5 minutes)
A. Build Background Knowledge: Explain to students that today they will be learning how to subtract two-digit numbers within 20. Ask students to give examples of subtraction (e.g. 10-3=7).
B. Activate Prior Knowledge: Ask students who may have done subtraction before to show the class how to subtract two-digit numbers within 20. Allow for volunteers to demonstrate.

II. Guided Instruction (15 minutes)
A. Model: Show students how to solve two-digit subtraction problems. Demonstrate how to use a number line and/or the regrouping process to subtract numbers within 20. Allow for the students to ask questions for further clarification.
B. Check for Understanding: Ask students to give answers and explain the reasons why they chose their particular answer. Additionally, use questioning to assess student understanding (e.g. "How did you come up with this answer?" "What did you do first?").

III. Adaptation (5 minutes)
A. Repeat Modeling: Allow for students to have a 'practice round' in order to practice reaching the correct answer by modeling what was previously done.

IV. Application & Reflection (10 minutes)
A. Group Activity: Divide students into pairs and provide each pair with a task or worksheet. Have them work together to solve two-digit subtraction problems. Instruct students to explain their thinking and reasoning out loud with their partner in order to ensure understanding.
B. Reflection: Ask students to share their strategies and strategies of their partner with the class. Ask one student to explain the strategy they used when subtracting two-digit numbers within 20. Encourage other students to provide input and feedback.

V. Closure (5 minutes)
A. Summary: Summarize the main points of the lesson by reiterating the goal and what was learned. Ask a student to give an example of two-digit subtraction.

VI. Assessment (3 minutes)
A. Exit Ticket: As students leave the lesson, provide them with an exit ticket to assess the knowledge that was gained from the lesson.