Cogito
Grade 2 Math · Chapter 1 · Lesson 6
Find the Missing Number
The unknown can sit anywhere in the sentence.
10 problems · about 13 minutes · 2.OA.A.1, 2.OA.B.2
What this lesson teaches
The child solves equations within 20 with the unknown in any position, including start unknown.
- The missing number can be anywhere in the sentence.
- The equal sign means both sides are worth the same.
- Check an answer by putting it back in.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
2 problemsReview — 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.
Review — Take From Ten to Subtract: 15 − 8 = ?
Answer 7
Why 2 + 5 = 7.
Warm Up
Straightforward practice. Get the method working first.
4 problems? + 8 = 15
Answer 7
Why 7 + 8 = 15.
What does the equal sign mean?
Answer Both sides are worth the same.
Why It means the two sides balance, whichever side the total is on.
9 + ? = 16
Answer 7
Why What goes with 9 to make 16?
? + 5 = 13
Answer 8
Why The whole is 13.
Build It Up
The same ideas with more to keep track of.
2 problems17 − ? = 9
Answer 8
Why How far from 9 up to 17?
14 = 6 + ?
Answer 8
Why The total is written first here.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
2 problemsThe Jar: Ana put 7 more marbles in a jar. Now there are 16. How many were in it before?
Answer 9 marbles
Why 9 + 7 = 16, so there were 9 to start with.
The Bookshelf: Leo took 8 books off a shelf. 7 are left. How many books were there at the start?
Answer 15 books
Why 8 + 7 = 15 books at the start.