Lesson 4

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 add or subtract multiples of 10.

When students share strategies based on adding or subtracting the digits in the tens place and explain why this method works, they are looking for and making use of the base-ten system and expressing regularity in repeated reasoning (MP7, MP8).

• 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 use the same method but would explain it differently?”
• “Did anyone approach the problem in a different way?”

The purpose of this activity is for students to solve two story problems involving adding or subtracting multiples of 10. Students are presented with a familiar context from a previous lesson in which students counted cubes in different bags. The quantities of cubes are described using representations that students are familiar with, and students add or subtract in a way that makes sense to them. The familiar context and representations are used to help students make sense of adding and subtracting tens. In the synthesis, students compare and connect the different representations used for each problem (MP2).

• Groups of 2
• Give students access to connecting cubes in towers of 10 and singles and double 10-frames.

• Read the task statement.
• 7 minutes: independent work time
• 3 minutes: partner discussion
• Monitor for students who show:
  • towers of 10
  • base-ten drawings
  • __ tens and __ tens
  • expressions or equations

If students attempt to add or subtract using single cubes (or representations of single cubes), consider asking:

• “How are you planning to find out how many cubes there are now?”
• “How could you use what we've learned about tens to solve this problem?”

• Invite previously identified students to share their representations.
• “How are these representations the same? How are they different?”

The purpose of this activity is for students to practice adding and subtracting multiples of 10 from multiples of 10. Students may use any method that makes sense to them, such as using connecting cubes in towers of 10, making base-ten drawings, or adding or subtracting the single-digit number of tens. When students notice they can add tens to tens just like they add single-digit numbers, they look for and make use of the base-ten structure of numbers (MP7).

• Groups of 2
• Give students access to connecting cubes in towers of 10.
• “Now you will practice adding and subtracting with tens.”

• 5 minutes: independent work time
• 5 minutes: partner work time

If students count on by one or do not find the numbers that make each equation true, but are able to complete the base-ten drawings and addition and subtraction statements, consider asking:

• “How did you find the number to make this equation true? How can you prove that the equation is true?”
• “How could you think about this equation as adding (or subtracting) tens?”

• Share responses and different methods used for each problem.