Operations and Algebraic Thinking: Mental Strategies CCSS 2.OA.2 Facts & Worksheets

Use this worksheet to help the student learn how to mentally add and subtract numbers within 20.

See the fact file below for more information on mental strategies or alternatively, you can download our 25-page Operations and Algebraic Thinking: Mental Strategies worksheet pack to utilise within the classroom or home environment.

Key Facts & Information

Learning Objective

At the end of the lesson, the student will be able to learn how to mentally add and subtract numbers within 20. This will help the student memorize all the sums of two one-digit numbers.

A Note for the Teacher

When mentally adding and subtracting, take note of the following:

• Adding means having a greater value, and subtracting means having a smaller value.
• Dealing mentally with two one-digit numbers does not have an answer that is greater than 20.
• Remind the student that they may count to solve mentally. 

Theory

Solving basic mathematical equations does not always necessarily need a pencil and paper. Mentally calculating the sum and difference is a skill.

There are various mental strategies in adding and subtracting within 20.

You may do the following:

• Memorize.

The simplest way to easily solve addition and subtraction within 20 is by memorizing their sums and differences. A good way to memorize sums and differences is by creating an addition and subtraction chart.

• Count on.

Since we are handling mostly single-digit numbers, it won’t take too much time to just count on whether forward or backward. 

For example, 4 + 2 means we count on 2 numbers forward after 4.

• Make 10.

We know that anything added to 10 adds 1 to the tens place of a number. Thus, it is easier when we’re working with 10. 

For example, 8 + 3 means we can make 10 with 8 + 2, then add the remaining value from 3, which was 1. Thus, 8 + 3 = 11.

• Remember 9 only needs 1 to be 10.

Since it is easier to handle equations with the number 10, we can easily add 1 to 9 from the addend. 

For example, 9 + 6 means we take 1 from 6 making it 5. The 1 we took from 6 will be added to 9 making 10. Thus, 9 + 6 = 10 + 5 = 15.

• Work with doubles.

Just like memorization, we can breeze through numbers added to themselves by just remembering their sums. (1+1=2, 2+2=4, 3+3=6…)

These strategies can then be applied to subtraction as well.

Operations and Algebraic Thinking: Mental Strategies CCSS 2.OA.2 Worksheets

This is a fantastic bundle that includes everything you need to know about Operations and Algebraic Thinking: Mental Strategies across 25 in-depth pages. These are ready-to-use worksheets that align with the Common Core code 2.OA.2 for Mental Strategies.

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Complete List of Included Worksheets

• A lesson plan
• Warm-up activity
• Math theory explained
• Assisted learning activities
• Independent learning activities
• Extension activities and games
• Answer keys

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