# Lesson 12

Ecuaciones con números desconocidos

### Lesson Purpose

### Lesson Narrative

In previous lessons, students found the value of sums and differences within 100, represented their thinking on the number line, and compared different methods.

In this lesson, students find the number that makes addition and subtraction equations true within 100 in equations with unknowns in all positions. They continue to think about the relationships between the numbers in each equation and reason about how they can use the structure of the number line, the relationship between addition and subtraction, and their understanding of place value to find the number that makes each equation true (MP7). The context of the first activity encourages all students to reason with the number line. In the second activity, students may use the methods and representations that make the most sense to them. The work of this lesson helps students make sense of the way the number line can be used to represent and solve story problems which will be useful in upcoming lessons.

- Representation

- MLR8

### Learning Goals

Teacher Facing

- Write equations and represent sums and differences on a number line.

### Student Facing

- Escribamos ecuaciones con el signo ? para representar el número desconocido.

### Required Materials

Materials to Copy

- Number Line to 100

### Required Preparation

### Lesson Timeline

Warm-up | 10 min |

Activity 1 | 20 min |

Activity 2 | 15 min |

Lesson Synthesis | 10 min |

Cool-down | 5 min |

### Teacher Reflection Questions

### Suggested Centers

- Number Puzzles: Addition and Subtraction (1–4), Stage 4: Within 100 with Composing (Addressing)
- Number Line Scoot (2–3), Stage 1: Twos, Fives, and Tens (Addressing)

### Print Formatted Materials

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Student Task Statements | docx | |

Lesson Cover Page | docx | |

Cool Down | Log In | |

Teacher Guide | Log In | |

Teacher Presentation Materials | docx | |

Blackline Masters | zip |