Lesson 2

The purpose of this warm-up is to elicit strategies and understandings students have for making expressions that are equivalent, but easier to find. These understandings help students develop fluency and will be helpful later in this lesson when students will need to be able to find missing digits that make equivalent expressions.

• Display one statement.
• “Give me a signal when you know whether the statement is true and can explain how you know.”
• 1 minute: quiet think time

• Share and record answers and strategies.
• Repeat with each statement.

• “How can the second statement help you with the last one?”

The purpose of this activity is for students to add or subtract to find unknown addends within 20. Students will find unknown addends when the total is 20. The known addends may encourage students to think about ways to make a ten and to use the sums and differences that they know (MP7).

•    Create a set of cards for each group of 2.

• Groups of 2
• Give each group a set of cards.

• “Let’s play another game to find unknown addends.”
• Select a volunteer to be your partner.
• “Partner A will draw a card and hold it on their forehead without looking.”
• Prompt the volunteer to demonstrate.
• “Partner B names a number that you can add to the number on the card to get to 20.”
• Think aloud finding and naming the number.
• “Partner A then names the number they are holding on their head.”
• Prompt the volunteer to find the number.
• “Check your work and write an equation to represent how you found an unknown addend in the turn. Share your equation with your partner. Switch roles and play again.”
• 12 minutes: partner work time
• Monitor for students who:
  • use place value reasoning or look to make 10
  • use known addition or subtraction facts
  • make equivalent, but easier sums or differences

• “Mai thought of trying to make 2 tens. What number needs to be added to 12 to make 2 tens?”
• “What other methods did you use to find the unknown addend?”
• Invite previously identified students to share methods.
• “Pull out the number cards for 0–9, as you will use them in the next activity.”

The purpose of this activity is for students to create different expressions that have the same value. The activity encourages students to decompose numbers and to consider how they may create sums that are equivalent but easier to find as a mental strategy (MP7). Students use the number cards 0–9 from the previous activity to help them figure out the missing number in the equations.

• Each group of 2 needs the digit cards 0–9 from the card set used in the previous activity.

• Groups of 2
• Give each group a copy of the puzzles.

• “You are going to use your cards to complete number puzzles. You may only use numbers 0 through 9. You may only use each number one time to make all the equations true.”
• 10 minutes: partner work time

• “Which equations could only be made true using one of your digits?”
• “Which equations have many ways you could make them true?”

• “What did you do when you ended with digits that you could not use to complete a true equation?” (We thought of other numbers that would work with what we had left. We completed the equations that had only 1 unknown first.)