Adding with Tens

Overview

Students solve addition and subtraction problems using part-part-whole reasoning for 2-digit numbers. The lesson includes a short modelling phase, guided practice, and independent word-problem practice focused on adding 2-digit numbers (some with 3 addends).

Learning intentions

• Students will partition 2-digit numbers into tens and ones to help them solve addition problems.
• Students will represent problems as number sentences and explain how the parts combine to make the whole.
• Students will use mental strategies and informal jottings (e.g., part-part-whole) to find sums.
• Students will check their answers using the context of the problem.

Success criteria

• I can write a number sentence for an addition word problem.
• I can split each 2-digit number into tens and ones and add to find the total.
• I can explain how my parts make the whole (tens and ones).
• I can solve independently and show my thinking (diagram, number line, or part-part-whole).

Curriculum links

• Number — add and subtract one- and two-digit numbers using number sentences and part-part-whole reasoning.
• Number — use mental strategies, informal written jottings, and recognise the role of zero where needed.
• Algebra — recall addition facts to 20 to support efficient solving and related subtraction facts.

Lesson structure (20 minutes)

1. 0–3 min · Warm-up (mental + vocab). Teacher writes: 34 + 20 and asks, “What changes and what stays the same?” Students discuss how adding 20 affects tens and leaves ones.

2. 3–7 min · Teacher model (part-part-whole). Teacher models a word problem with two 2-digit addends (e.g., “Jade has 34 stickers and gets 58 more.”) using a part-part-whole diagram: tens + tens, ones + ones, then total; students copy the structure into books.

3. 7–10 min · Guided practice (think-aloud). Teacher shows two problems one at a time and works through one with students’ input:

• 46 + 23
• 35 + 27 (students suggest a strategy) Students write number sentences and a brief jottings plan (tens and ones).

4. 10–17 min · Independent practice (10 word problems). Teacher explains expectations: solve each, write the number sentence, and show one strategy (part-part-whole or tens/ones). Students complete 10 questions using 2-digit numbers; teacher circulates and prompts without giving answers.

5. 17–20 min · Share + quick check. Teacher selects 2–3 students to share a strategy for one problem (especially a 3-addend question). Students complete a 1-minute exit check: “Solve: 42 + 29” and show tens/ones thinking.

Resources

• Student books and pencils
• Part-part-whole diagram template (on board or handout)
• Base-ten blocks or place-value cards (tens and ones) for teacher use
• 10-question independent practice sheet (printed or on board)
• Whiteboard markers and erasers
• Timer visible to students

Assessment

• Observe during circulation: can students partition into tens/ones and connect parts to whole?
• Check independent work for: correct number sentence, correct sum, and at least one clear strategy/jotting.
• Exit check: 42 + 29—confirm students’ method aligns with tens/ones reasoning.

Differentiation

• Support (for students needing scaffolds):
• Provide a sentence starter: “I split __ into __ tens and __ ones. Then I add __ ones and __ tens.”
• Offer a partially completed example for one problem (number sentence frame and place-value headings).
• Allow using base-ten blocks or place-value cards for the independent set.
• Target teaching moves (during circulation):
• Ask: “What are the tens in this number?” “What happens to the ones?”
• Encourage checking: “Does your answer make sense with the story?”
• Extensions (for advanced learners):
• Add an extra step: write two different strategies for one question (e.g., tens/ones and near doubles or bridging to 10 where helpful).
• Solve a bonus “missing part” challenge: “If Tom has 34 + 58 +? = 105, what is the missing number?”
• EAL/SEN considerations:
• Use consistent language for tens and ones; highlight addends and totals with colours on the worksheet.
• Keep word problems short; underline key words like “altogether”, “more”, “then”, “and”.

Word problem set (Independent Practice: 10 questions)

Write a number sentence for each problem and then find the total.

1. John had 34 sheep in one paddock and 58 sheep in another. How many sheep does he have altogether?
2. Lily collected 27 seashells from one beach and 36 from another beach. How many seashells altogether?
3. Mia had 45 marbles. She won 23 more marbles. How many marbles does she have now?
4. A bus driver counted 26 passengers in the morning. Later, 31 more passengers got on. How many passengers in total?
5. There were 39 birds in the park. Then 24 more birds flew in. How many birds are there altogether?
6. Sam has 18 stickers. He gets 26 stickers on Monday and 17 stickers on Tuesday. How many stickers does he have altogether?
7. Ava bought a book for 28 dollars. She also bought a pencil case for 19 dollars. How much money did she spend altogether?
8. Oliver had 57 blocks. He gave away 12 blocks to a friend. How many blocks does Oliver have left?
9. Three friends shared 2 tins of paint. Tin A has 43 ml and Tin B has 29 ml. How much paint is there altogether?
10. The school library had 24 picture books. They bought 38 more picture books and 16 story books. How many books did the library have altogether?

(Teacher note: Questions 6 and 10 include 3 addends; question 8 includes subtraction to strengthen understanding of the relationship between parts and whole.)