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  structure of teen numbers. Students can write many different equations to represent each problem or how they solved it. It’s important that students are able to relate their equations to the story problem and explain their work (MP2, MP4).

• Invite previously identified students to share their equations for Priya’s problem.
• If no student writes a subtraction equation, display .
• “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 unknown number. The subtraction equation 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 both numbers to add 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).

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

• Share solutions for each problem.
• “How are and related?” (One is an addition problem and one is a subtraction problem, but you can use addition or subtraction for either one. For , you can subtract or change it to . For , you can add or change it to . Both have an unknown value of 10.)