Cogito
Grade 2 Math · Chapter 1 · Lesson 7
Choosing a Strategy
Look at the numbers before you start.
10 problems · about 13 minutes · 2.OA.B.2
What this lesson teaches
The child selects an efficient mental strategy for a given fact within 20 and explains the choice.
- Look at the numbers before you start working.
- Equal numbers mean a double. An 8 or 9 means make ten.
- More than one strategy can be right. Counting one by one is the slow one.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
2 problemsReview — Find the Missing Number: ? + 8 = 15
Answer 7
Why 7 + 8 = 15.
Review — Addition and Subtraction Together: You know 8 + 5 = 13. What is 13 − 5?
Answer 8
Why 13 − 5 = 8, because 8 and 5 make 13.
Warm Up
Straightforward practice. Get the method working first.
4 problems9 + 7 = ?
Answer 16
Why 9 + 1 + 6 = 16.
Which strategy fits 6 + 7 best?
Answer Near doubles.
Why 6 + 6 = 12, and one more makes 13.
Which strategy fits 9 + 5 best?
Answer Make ten.
Why The 9 is one away from ten.
Which strategy fits 7 + 7 best?
Answer Doubles.
Why Both numbers are the same.
Build It Up
The same ideas with more to keep track of.
2 problems8 + 6 = ?
Answer 14
Why Make ten, or double the 7 in the middle.
13 − 8 = ?
Answer 5
Why Take from the ten.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
2 problemsThe Shop: A pen costs 9 pence and a rubber costs 7. What do they cost together?
Answer 16 pence
Why 9 + 1 + 6 = 10 + 6 = 16.
The Garden: Ana planted 6 seeds and Leo planted 7. How many seeds did they plant?
Answer 13 seeds
Why 6 + 6 = 12, and one more makes 13.