The purpose of this activity is for students to solve addition and subtraction problems within 100 with the unknown in all positions. Students write equations with a ? for the unknown and find the number that makes the equations true. The mathematical context of each problem encourages students to use the number line to reason about what is unknown and how they may represent the problem with an equation (MP2).

• Invite students to share the answer to each riddle and display their number line.
• “Which riddles did you and your partner find to be most challenging? Explain.”

The purpose of this activity is for students to find the value of an unknown in addition and subtraction equations. Students can choose to find the unknown number using either operation and then represent their thinking on a number line. Listen for the ways students use the number line to make sense of the relationship between the numbers in each equation. Also listen for methods that show students are thinking about ways to use the structure of the number line and their understanding of place value (MP7).

• Invite previously selected students to share how they found the number that makes true.
• Consider asking:
  • “How does _____’s number line show the numbers that we knew? How does it show the unknown number?”
  • “How are these methods the same? How are they different?”
• As time permits, continue with other equations.