Subtract Within 20

Overview

In Lesson 16 of 30 in Building Mathematical Thinking, students develop strategies for subtracting within 20. They connect subtraction to taking away, counting back, and related addition facts, building from concrete models toward equations and explaining their thinking.

Learning intentions

• Students will be able to subtract within 20.
• Students will be able to model subtraction using counters, drawings, or a number line.
• Students will be able to explain a subtraction strategy using words, gestures, or an equation.
• Students will be able to use a related addition fact to check a subtraction answer.

Success criteria

• I can solve at least four subtraction problems within 20.
• I can use counters, a drawing, a number line, or another strategy to show my thinking.
• I can explain how I found the difference.
• I can check a subtraction answer with addition when I can.

Curriculum links

• Operations and Algebraic Thinking — fluently add and subtract within 5.
• Operations and Algebraic Thinking — relate counting to addition and subtraction.
• Operations and Algebraic Thinking — add and subtract within 20, using strategies such as counting on, decomposing numbers, and related addition facts.
• Operations and Algebraic Thinking — find the number that makes 10 when added to a given number, supporting subtraction strategies that bridge through 10.

Lesson structure (30 minutes)

1. 0–4 min · Hook and connect. Open with the subtraction hook and learning target slides and show 8 counters; hide or remove 3 while asking, “How many are left? How do you know?” Students quietly solve, show an answer with fingers or counters, and share a strategy with a partner.

2. 4–10 min · Model strategies. Use the strategy modeling slides to model (13-4) with counters, a take-away drawing, and a number line. Think aloud: “I start at 13 and count back 4: 12, 11, 10, 9,” then model checking with (9+4=13). Students act out each step with counters or a personal number line and identify which strategy feels useful.

3. 10–20 min · Guided practice. Display the problems and visual prompts on the guided practice slides and distribute the subtract-within-20 practice sheet. Students solve four to six problems, choosing counters, a number line, take-away pictures, or a known fact; pause after each pair of problems for students to compare methods. Confer with individuals and ask, “What number did you start with?” and “How many did you take away?”

4. 20–26 min · Partner strategy talk. Return to the partner discussion and challenge slides and assign partners within the group of 10. Each student chooses one worksheet problem, shows the model or strategy, and explains the answer; partners listen, repeat the equation, and check it with addition if possible.

5. 26–30 min · Exit check and share. Use the plenary and exit prompt slide and ask students to solve (15-6) independently on the back of the worksheet or on an individual response board. Students show a model or strategy, state the answer, and share one way they checked or could check it.

Resources

• subtraction strategy and practice slide deck
• subtract-within-20 practice sheet
• Two-color counters or small connecting cubes
• Individual number lines from 0 to 20
• Pencils, crayons, and erasers
• Mini whiteboards or individual response boards
• Large-print subtraction symbols and equation cards
• Optional ten-frames and take-away picture mats

Assessment

• During modeling, check whether students can represent the starting number and remove the correct quantity.
• During guided practice, record whether each student solves at least four problems and note the strategy used, accuracy, and ability to explain.
• Use the final (15-6) response as an exit check. Sort students into groups for the next lesson: independent within 20, supported within 20, or additional practice within 10.

Differentiation

• Support: Begin with numbers within 5 or 10, use counters and take-away visuals, and provide a number line with marked starting points. Present one step at a time: “Build the first number. Take away. Count what remains.”
• Language and communication: Accept spoken answers, pointing, gestures, drawings, or selecting an equation. Offer the sentence frame, “I started with __. I took away __. __ are left.”
• Dyslexia-friendly access: Use large, uncluttered symbols, high-contrast print, color-code the starting number and amount taken away, read all directions aloud, and allow students to respond verbally or with manipulatives. Avoid requiring students to copy lengthy explanations.
• Advanced learners: Invite students to solve a problem in two ways, create a subtraction story within 20, or use a related addition fact to verify the answer. Ask, “Can you find another subtraction problem with the same answer?”

Extension

• Students create a subtraction equation within 20, represent it with a drawing or counters, and trade with a partner to solve.
• Students investigate whether changing the order of the numbers always gives the same subtraction answer and explain what they notice.