Addition and Subtraction: Facts to 20 and Strategies to 100
Warm-up
Number talk: 48 + 37. Students solve mentally. Collect strategies: decompose tens and ones; friendly numbers; bridging. Record each strategy on the board. Compare: which is fastest? Which is clearest? All correct strategies are valid; flexibility is the goal.
Explore
Strategy stations: (1) Hundred chart: students colour an addition path (e.g., 48 + 37: start at 48, jump +30 to 78, jump +7 to 85). (2) Open number line: record the same problem as jumps. (3) Decompose: 48+37 = (40+30) + (8+7) = 70+15 = 85. All three representations; compare.
Formalize
Focus on adding up to subtract: 73 - 46 on an open number line. Start at 46. Jump to 50 (+4). Jump to 73 (+23). Total jumps: 4 + 23 = 27. Why does this work? Because 46 + 27 = 73, and subtraction asks what do you add to get from one number to another.
Addition and Subtraction: Facts to 20 and Strategies to 100
Estimation: before computing 67 + 28, estimate using benchmarks. 67 is close to 70; 28 is close to 30. 70 + 30 = 100. So the answer is close to 100. Compute and check: 95. Estimation catches errors before they become wrong answers.
Practice
Students solve 6 addition and 4 subtraction problems to 100, recording their strategy for each. Exit ticket: 65 + 28 using any strategy. Write the strategy name and the answer.
Exit ticket
Students solve 6 addition and 4 subtraction problems to 100, recording their strategy for each. Exit ticket: 65 + 28 using any strategy. Write the strategy name and the answer.
Step 1: Start at 38. Add the tens of 25 first: one jump of 20 lands at 58. Draw the jump and label it +20.
Step 2: Split the remaining 5 strategically: jump +2 to reach the friendly number 60.
Step 3: Jump the last +3: 60 → 63. So 38 + 25 = 63.
Step 4: Record the whole path: 38 → (+20) → 58 → (+2) → 60 → (+3) → 63. Three easy jumps replaced one hard calculation.
Why this beats column addition here: no regrouping rules to memorize — the strategy is visible, checkable, and each landing point makes sense. Column algorithms come later; understanding comes first.
Step 1: Reframe: 62 − 38 asks "how far is it from 38 up to 62?" Finding a gap by climbing is often easier than taking away.
Step 2: Climb in friendly stages: 38 + 2 = 40 (note the 2). 40 + 20 = 60 (note the 20). 60 + 2 = 62 (note the 2).
Step 3: Add the climbs: 2 + 20 + 2 = 24. So 62 − 38 = 24.
Step 4: Check by estimating: 62 − 38 should be close to 60 − 40 = 20. Our 24 is nearby ✓.
Watch for: students who lose one of the recorded jumps. Have them draw the number line and write each jump ON the arc until the bookkeeping is automatic.
Step 1: Notice 29 is annoying but 30 is friendly. Add 30 instead: 47 + 30 = 77. That was easy — but we added 1 too many.
Step 2: Give the extra 1 back: 77 − 1 = 76. So 47 + 29 = 76.
Step 3: Say the strategy's rule: "add a friendlier number, then repair the damage." The repair is always the difference between what you used (30) and the truth (29).
Step 4: Stress-test understanding with subtraction, where the repair flips: 53 − 29 → take 30 (getting 23), but you took away 1 TOO MANY, so give 1 back: 24. Students who repair in the wrong direction (22) are following a rule; students who ask "did I take away too much or too little?" are reasoning. Require the question, not the rule.