Lesson 12
Mentally Add and Subtract Tens (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
- Let’s add and subtract tens in our heads.
Required Materials
Materials to Gather
Materials to Copy
- Write the Number Stage 2 Gameboard
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?