Add and subtract within 100

What you will learn

• Add and subtract within 100

Models used: number sentences, stories, bar models.

Before you start

• Find the missing part

Make a ten, then keep going

We have 8 counters and add 5 more. How many are there altogether? (8 + 5)

1. Each counter stands for one object. The separate groups are parts; count all the groups to find the whole.

   Let’s make 10 first. 8 needs 2 more to make 10.

   We can regroup or split an amount while keeping its value. That lets us work with one useful part at a time.

   85

2. So we split 5 into 2 and 3.

3. Each counter stands for one object. The separate groups are parts; count all the groups to find the whole.

   8 + 2 = 10. Then 10 + 3 = 13.

   8 + 2 = 10 · 10 + 3 = 13

   85

4. Each counter stands for one object. The separate groups are parts; count all the groups to find the whole.

   So 8 + 5 = 13.

   8 + 5 = 13

   85

Worked example: A new example, together

What is 17 + 58?

1. Start at 17 on the number line.

2. Add the tens first: + 50. Then the ones: + 8.

   17 + 50 = 67 · 67 + 8 = 75

3. We land on 75.

4. Use the picture and the steps we have just followed to finish this question.

   What is 17 + 58?

   75

5. So 17 + 58 = 75.

   17 + 58 = 75

Common mistakes to discuss

• Losing the exchange. Ten ones become one ten, which must be recorded in the next column. In 52 − 27 the ones need an exchange, so the answer is 25, not 35 from taking 2 from 7.

Check your understanding

• Add and subtract within 100
• explain an addition or subtraction strategy without changing its quantities
• check the answer against the original quantities and units

Explain your reasoning to an adult, then try a fresh question in the interactive lesson. A result from one attempt is not proof of lasting mastery.