Lesson 10

• 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 can you use problem one to help you find the difference in problem two?” (If \(10 + 4 = 14\), then I know that \(14 - 4 = 10\).)
• “Did anyone approach the problem in a different way?”

The purpose of this activity is to elicit methods students have for solving story problems involving addition and subtraction with teen numbers. Students are presented with story problem types that are familiar to them to allow for discussion about methods they used to find the answer. Students solve the problems in any way that makes sense to them. They may build values and add-on or take-away, or use what they have learned about the \(10 + n\) structure of teen numbers. Students write equations; they can write many different equations to represent the problem or how they solved it. It is important that students are able to relate their equations to the story problem and explain their work (MP2, MP4).

• Groups of 2
• Give students access to double 10-frames and connecting cubes or two-color counters.
• “Many people have the hobby of collecting things. Do any of you collect something?”
• Share responses.
• “What are some things that you know people collect, or that you might like to collect?” (baseball cards, marbles, rocks)
• 30 seconds: quiet think time
• 1 minute: partner discussion
• Share responses.
• “Let’s solve some story problems about collections.”

• Read the task statement.
• “Write two equations to match each of these stories.”
• 6 minutes: independent work time
• 4 minutes: partner discussion
• Monitor for a student who wrote an addition equation and one who wrote a subtraction equation for the problem about Priya.

• Invite previously identified students to share their equations for Priya’s problem.
• If no student writes a subtraction equation, display \(13 - 3 =\boxed{\phantom{10}}\).
• “How do the equations match the story problem? How are they related to each other?” (They show that the total number of comic books is 13 and that 3 is one part and 10 is the other part. They all show that 10 is the missing number. The subtraction equations takes 3 away from the 13 to find the other part. The addition equation adds 3 to some number to get 13.)
• “Which equation makes more sense to you? Why?” (Addition, because I know that to get from 3 to 13, I have to add 10. Subtraction, because I know what number to start with and how many to take away. I don't know what numbers to use in the addition equation.)

The purpose of this activity is for students to discuss the relationship between addition and subtraction equations involving teen numbers. Students find the value that makes the addition and subtraction equations true with the unknown in all positions. Students may choose to use objects to represent the problems and find the value that makes the equation true (MP5).

• Groups of 2
• Give students access to double 10-frames and connecting cubes or two-color counters.
• Read the problem about Mai.
• 30 seconds: quiet think time
• 1 minute: partner discussion
• Share responses.

• Read the task statement.
• 4 minutes: independent work time
• 4 minutes: partner discussion

If students take away using drawings for each equation, consider asking:

• “How did you find the missing value?”
• “How could the double 10-frame help you find the missing value?”

• Share solutions for each problem.
• “How are \(\boxed{\phantom3} = 13 - 3\) and \(2 + \boxed{\phantom 3} = 12\) related?” (One is an addition problem and one is a subtraction problem, but you can use addition or subtraction for either one. For \(\boxed{\phantom 3} = 13 - 3\) you can subtract or change it to \(3+ \boxed{\phantom3} = 13\). For \(2 + \boxed{\phantom3} = 12\) you can add or change it to \(12 - 2=\boxed{\phantom3} \). Both have a missing value of 10.)