Lesson 8

How Many in the Bag

Est. Class Sessions: 2–3

Summarizing the Lesson

After the last round, pose and demonstrate the following problem with a bag and counters:

X
  • There are 20 counters in the bag. I take 11 out. How many are left in the bag?

Display the following number sentence:

SG_Mini

Choose a strategy from the Invented Strategies chart and ask students to solve the problem using this strategy. Have a volunteer share his or her solution. See Sample Dialog 2 for a sample discussion where students use strategies that involve using doubles, ten frames, and decomposing numbers to solve 20 − 11.

X

Teacher: Let's look at 20 − 11.

Ana: I solved it using doubles. I know 10 + 10 is 20, so
20 − 10 is 10. But it's 20 − 11, so I needed to take one more off. The answer is 9.

Teacher: Good. Did anyone solve it a different way?

Grace: I did. I took 11 apart in my mind. I made it 10 + 1. So when I take away 11 counters I take away 10 and then 1 more. The answer is 9. My number sentence is
20 − 11 = 9.

Teacher: Good, Grace. When you took 11 apart in your mind, did you have a picture in your mind?

Grace: Yes, a ten frame. The 10 went into one ten frame, and the 1 went into a different ten frame.

Teacher: Is anyone else able to make a math picture in their heads like Grace did?

Ming: I pictured a doubles picture. My doubles picture was 10 and then 20.

Roberto: I can picture a number line.

Teacher: That's a good thing about our minds, isn't it? When we work with something for awhile, like cubes or ten frames or doubles pictures or the number line, then sometimes we can work a problem when we don't have those things with us. We can see them in our heads and it helps us to figure out the problem.

Next, change the format of the question. Demonstrate the following problem:

SG_Mini

Ask:

X
  • There are 20 counters in the bag. I take some out. Now there are 14 left in the bag. How many did I take out?

Choose another strategy from the chart, possibly one that involves using fact families, and ask students to use it to solve the problem. Have a volunteer share his or her solution. Ask students to identify what the numbers in the number sentence mean. Continue practicing with other subtraction situations involving 20 counters in the bag, an unknown number of counters taken out, and a known number left in the bag.

Assign the It Is in the Bag Homework Master to provide more practice with subtraction situations where students must find unknown numbers.

Student Guide:
Student Activity Book:
Master: