Cogito
Grade 2 Math · Chapter 1 · Lesson 5
Addition and Subtraction Together
One fact family, four number sentences.
10 problems · about 12 minutes · 2.OA.B.2, 2.OA.A.1
What this lesson teaches
The child writes the four related equations for a part-part-whole relationship within 20.
- Two parts and a whole make four number sentences.
- A subtraction question is asking for the missing part.
- Knowing one fact hands you the other three.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
2 problemsReview — Take From Ten to Subtract: 15 − 8 = ?
Answer 7
Why 2 + 5 = 7.
Review — Make Ten to Add: 9 + 8 = ?
Answer 17
Why 9 + 1 + 7 = 10 + 7 = 17.
Warm Up
Straightforward practice. Get the method working first.
4 problemsYou know 8 + 5 = 13. What is 13 − 5?
Answer 8
Why 13 − 5 = 8, because 8 and 5 make 13.
How many number sentences come from parts of 6 and 9?
Answer 4
Why 6 + 9, 9 + 6, 15 − 6, and 15 − 9.
You know 7 + 6 = 13. What is 13 − 7?
Answer 6
Why The other part.
You know 9 + 8 = 17. What is 17 − 8?
Answer 9
Why The other part.
Build It Up
The same ideas with more to keep track of.
2 problemsParts of 5 and 9. What is the whole?
Answer 14
Why Add the two parts.
Whole 18, one part 9. What is the other part?
Answer 9
Why This one is a double.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
2 problemsThe Coat Rack: 14 coats hang on a rack. 6 are red. How many coats are not red?
Answer 8 coats
Why 6 + 8 = 14, so 14 − 6 = 8.
Lost Marbles: Leo had 17 marbles. Now he has 9. How many did he lose?
Answer 8 marbles
Why 9 + 8 = 17, so 17 − 9 = 8.