Lesson 12

The purpose of this Number Talk is to elicit strategies and understandings students have for adding and subtracting 10. These understandings help students develop fluency and will be helpful later in this lesson when students mentally add and subtract 10 from larger two-digit numbers.

• 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.

• “Did anyone approach the problem in a different way?”

The purpose of this activity is for students to learn stage 2 of the Write Numbers center. Like the last stage of this center, students take turns writing the next one, two, or three numbers. The player who writes the last number on the board wins. Students may choose to count forward or backward. In this stage, students count by 10 starting at numbers other than 10.

• Put each gameboard in a sheet protector.

• Groups of 2
• Give each group a gameboard and a dry erase marker.
• “We are going to learn a new way to do the Write Numbers center.”
• Display the gameboard.
• “You and your partner will practice writing numbers. Just like the last time we played this game, you will fill in the number path on the gameboard. You can decide to start with the smaller number and count forward, or start with the larger number and count backward. On each turn, you can decide whether you would like to write one, two, or three numbers on the gameboard. The person who writes the last number on the board is the winner.”
• “Today you will not write every number in the sequence. You will count by ten and write each number you say.”
• Demonstrate playing one round with the students.
• “Now you will play with your partner.”

• 10 minutes: partner work time

• “What do you notice about each number on the gameboard?” (They all have the same amount of ones. The tens go up by one in each number.)

The purpose of this activity is for students to practice adding and subtracting 10 mentally from any two-digit number. The problems are organized into two sets. In the first set, students add and subtract 10 from the same number. Students may notice a pattern and generalize that when you add or subtract 10, the number of tens increases by one or decreases by one and that the ones place does not change (MP7). In the second set, students work with a range of numbers including adding or subtracting multiple tens to develop fluency.

• Groups of 2
• “Today you will add or subtract 10 from two-digit numbers and find the value that makes each equation true. First, you will complete some equations. When you finish all of the equations, you will talk with your partner about the patterns you noticed, then each partner will write about it.”
• “Then, you will complete some more equations, but after each set of equations you will talk with your partner about what you notice then write about it.”

• 15 minutes: partner work time

If students find the values of the expressions by counting on or counting back by one, consider asking:

• “How did you find the number to make this equation true?”
• “How could you use what you know about tens and ones to find the number that makes the equation true?”

• “What patterns did you notice as you added or subtracted 10?” (Only the tens place changes. In some of the equations I was adding 2 tens or 3 tens so it was like adding tens from before. I can skip count to add or subtract tens quickly.)