# Lesson 12

Sumemos y restemos decenas mentalmente (optional)

### Lesson Purpose

The purpose of this lesson is for students to use base-ten understanding to mentally determine 10 more or 10 less than a two-digit number.

### Lesson Narrative

In previous lessons, students added and subtracted multiples of 10 from other multiples of 10 and added two-digit numbers and tens using connecting cubes in towers of 10 and singles, base-ten drawings, numbers, words, and expressions.

This lesson is optional as students may not need more practice developing fluency with adding or subtracting 10. In the first activity, students learn stage 2 of the Write Numbers center. In this stage, students count by ten forward or backward and write each number as they count. In the second activity, students notice patterns when adding and subtracting ten from any two-digit number.

• Representation
• MLR8

### Learning Goals

Teacher Facing

• Mentally find 10 more or 10 less than a number.

### Student Facing

• Sumemos y restemos decenas mentalmente.

### Required Materials

Materials to Gather

Materials to Copy

• Write the Number Stage 2 Gameboard, Spanish

### Required Preparation

Activity 1:

• Put each gameboard in a sheet protector.

### Lesson Timeline

 Warm-up 10 min Activity 1 20 min Activity 2 20 min Lesson Synthesis 10 min Cool-down 0 min

### Teacher Reflection Questions

Why is a strong understanding of the unit ten necessary in order to add or subtract 10 mentally?