Lesson 4

The purpose of this Number Talk is to elicit strategies and understandings students have for making a ten to add. When students look for ways to decompose an addend to make 10, students look for and make use of the structure of numbers and the properties of operations (MP7). These understandings help students develop fluency and will be helpful later in this lesson when students will need to be able to add within 20.

• Display one expression.
• “Give me a signal when you have an answer and can explain how you got it.”
• 1 minute: quiet think time

• Record answers and strategy.
• Keep expressions and work displayed.
• Repeat with each expression.

• “How does making 10 help in finding the value of the sums?” (It helped me because I was able to use facts I already know. I know \(7 + 3 = 10\) and I know \(10 + 3 = 13\).)

The purpose of this activity is for students to revisit stage 1 of the How Close center, which was first introduced in grade 1. Students pick 5 digit cards and then choose 3 to make an expression that has a value as close as possible to the target number, which is 20 in stage 1. Students may use a variety of strategies such as making a 10 to add two numbers or using the relationship between addition and subtraction to strategically choose 3 numbers (MP7).

• Groups of 2
• Give each group a set of cards. Have them remove the number 10 cards.
• Display the digit cards at the top of the student page.
• “We are going to play a game called How Close. Let’s play 1 round together.”
• “Look at the 5 cards. Think about which 3 cards you would choose to add together to get close to 20.”
• 1 minute: quiet think time
• 2-3 minutes: partner work time
• Share responses.
• “After you choose your 3 cards, you will write an equation showing the sum.”
• Demonstrate writing an equation to show the sum of the 3 cards that get closest to 20.
• “When you have your equation written, compare it with your partner’s to see who found a sum closer to 20.”
• “Then you will replace the cards you used and play again.”

• "Now play How Close with your partner."
• 10 minutes: partner work time
• Monitor for students who:
  • try to make 10 and then add the greatest number possible to it
  • add the 2 greatest numbers and then look for the other number needed to make 20

• “What methods did you use to find a sum close to 20?
• Invite previously selected students to share their strategies.

The purpose of this activity is for students to add and subtract within 20 using any method that makes sense to them. In the previous activity, students shared methods they used to add within 20. This activity gives students an opportunity to try some of the methods they discussed. In this activity, students also subtract within 20 and will discuss their methods in the synthesis.

• Groups of 2
• Give students access to connecting cubes.
• “Now we are going to add and subtract within 20. Think about how you can show your thinking so others can understand.”
• “You have time to work independently before sharing your thinking with a partner.”

• 5 minutes: independent work time
• 5 minutes: partner discussion
• Monitor for different methods used to find the value of \(17 - 9\). For example:
  • adding on to make 10 ( \(9 + 1 = 10\), \(10 + 7 = 17\), \(1 + 7 = 8\))
  • decomposing one number to get to 10 first ( \(17 - 7 = 10\), \(10 - 2 = 8\))

If students are counting all of the cubes by 1 or working with loose cubes, consider asking:
"How could you use towers of 10 to help you find the sums or differences?"

• Display 17 – 9.
• Select previously identified students to share their strategies.