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).

• “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 can 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 numbers in the equations.

• “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.)