# Add and Subtract within 20

Using mental strategies to add and subtract within 20 is a second grade, Common Core math skill: 2.OA.2. Below we show two videos that demonstrate each of these standards. Then, we provide a breakdown of the specific steps in the videos to help you teach your class.

Prior Learnings

Your students should be experienced in solving all types of addition and subtraction problems. Before, “addition” was separate from subtraction, and the problems were limited to one-step problems and numbers within 20 (1.OA.1).

Future Learnings

After learning these math skills, your students will be able to apply strategies to both one and two-step problems that involve the “four operations (addition, subtraction, multiplication, division).” Additionally, your students will learn how to represent an unknown using a letter (3.OA.8).

Common Core Standard: 2.OA.2 - Fluently add and subtract within 20 using mental strategies

Students who understand this principle can:

1. Correctly choose mental strategies to add and subtract within 20.
2. Quickly recall all sums of 2 one-digit numbers.
3. Fluently add and subtract within 20.

Video 1: Adding and Subtracting while Exercising

The video opens on a young woman in the middle of her workout routine. Her online instructor, Anaya, told her to do 15 squats, she’s already done 7. Because she knows her math facts, she knows how many more squats she needs to do.

1. 15 - 7 = 8
2. She has 8 more squats to complete before moving on to the next exercise.

The speaker explains why knowing how to mentally solve math problems quickly is important. Then she moves onto the next exercise—crunches.

1. Anaya tells her to do 9 crunches and then take a break.
2. After a break, she says to do 9 more crunches.
3. How many are those in total?
4. 9 + 9 = 18
5. So, the speaker has to do 18 crunches in total.

At the end of the video, the speaker tells students that they will learn to meantally add and subtract within 20 in this quest or lesson.

Video 2: Practice Adding and Subtracting with Boddle

The video starts by introducing the topic: adding and subtracting within 20. Boddle starts with the basics by asking students to solve simple addition and subtraction problems.

1. 2 microphones + 1 microphone = 3 microphones
2. 3 crowns - 2 crowns = 1 crown
3. 8 cameras - 4 cameras = 4 cameras
4. 2 teddy bears + 1 teddy bear + 5 teddy bears = 8 teddy bears

Next, Boddle flips it around and has students match the answer to the correct math equation. Doing so helps students learn to approach problems from multiple angles.

Which addition or subtraction problem makes the number on the left?

1. 6 = 4 +2
2. 2 = 7 - 5
3. 4 + 10 - 6
4. 9 = 1 + 8
5. 7 = 3 + 4

Afterwards, Boddle has students find the missing number in a set of equations.

1. 8 + __ = 2 + 9
a. Both sides must be the same.
b. First solve the side that doesn’t have a missing number.
c. 2 + 9 = 11, so the left side must be 11 as well.
8 + 3 = 11
2. 12 - 5 = 3 + __
a. First solve 12 - 5.
b. 12 - 5 = 7, so the right side must also be 7.