# Addition and Subtraction within 20

Understanding addition and subtraction within 20 is a first grade, Common Core math skill: 1.OA.1. Below we show two videos that demonstrate this standard. Then, we provide a breakdown of the specific steps in the videos to help you teach your class.

Prior Learnings

Your students should be familiar with the Kindergarten skill of understanding the number partners that equal 10 and knowing all decompositions (e.g. 5=4+1, 5=2+3) of numbers below 10. This skill builds a foundation for strategy development and understanding of place values and properties of operations (K.OA.3-4). Your students should also understand that ten 1’s plus more 1’s are considered “teens” (K.NBT.1).

Future Learnings

Understanding how to perform addition and subtraction within 20 will enable your students to perform similar skills up to 100, eventually extending those skills to work with larger numbers and solve two-step word problems (2.OA.1). They will also be able to apply this skill with problems in a variety of contexts involving length, picture graphs and bar graphs (2.NBT.5).

Common Core Standard: 1.OA.1 - Use addition and subtraction within 20 to solve word problems involving situations of adding to, taking from, putting together, taking apart, and comparing, with unknowns in all positions.

Students who understand this principle can:

1. Use objects and drawings to illustrate word problems with situations like adding, removing, combining, taking apart, or comparison.
2. Write equations involving adding, removing, combining, taking apart, or comparison situations involving unknown numbers that change positions.
3. Explain how an equation represents one of the above situations.
4. Solve word problems that represent one of the above situations.

Video 1: Adding and Subtracting with Mr. Robot

The video presents four word problems with addition and subtraction problems for your students to follow along with. Mr. Robot helps the teacher in the video clear the board and engages with the students.

The first word problem states that “Sam has 4 hats and Josh has 8 hats. How many hats do they have altogether?”

1. The video shows how many hats each has. Your students can draw their own hats on a piece of paper.
2. The video counts how many hats Sam has, and then it counts how many hats Josh has.
a. “All together” means putting the groups of items together; it's also called adding.
3. The video uses a strategy called “counting on,” which means you will start with one of the groups and count up from its total.
4. Starts from the group of 8 and counts each of Sam’s hats, ending with 12 hats.’

The second word problem states that “Henry has 4 fewer cats than John. John has 9 cats. How many cats does Henry have?”

1. The word “fewer” means having a smaller amount. The word helps your students know they will need to subtract to get the answer.
2. Since Henry has 4 fewer, the video crosses 4 cats off of John’s nine cats.
3. It counts John’s remaining cats and ends up with 5 cats.
4. Henry has 5 cats.

The third word problem states that “Sam has 3 more pumpkins than Jim. Jim has 6 pumpkins. How many pumpkins does Sam have?”

1. The number of pumpkins Jim has is known (6), but the number Sam has is unknown.
2. Since Sam has 3 more, 3 more pumpkins are added.
3. The video counts on from Jim’s 6 pumpkins, ending with 9.
4. Sam has 9 pumpkins.

The fourth word problem states that “Cindy has 7 cakes. Suzy has 9. How many more cakes does Suzy have than Cindy?”

1. In this problem, you will use subtraction to find the answer.
2. The 9 cakes Suzy has are drawn.
3. It then crosses out the 7 cakes Cindy has and counts how many are left.
4. 2 cakes are left.
5. Suzy has two more cakes than Cindy does.

Video 2: Adding and Subtracting Flowers and Balloons

In the video, Boddle starts with 5 yellow flowers. She then gives them to her friend, Peter. As Peter walks in the park, he finds 2 more flowers. Boddle asks your students how many flowers Peter has in total now.

1. Boddle counts how many flowers she gave him, ending with 5.
2. Peter found 2 more flowers.
3. The video counts all the flowers, ending with 7.
4. Peter has 7 flowers in total.

The video gives another addition problem for your students to solve, asking “What is 8 + 4?”

1. The video counts out 8 cars.
2. Then 4 more cars are placed; Boddle asks your students how many cars there are.
3. There are 12 cars in total.

The second part of the video covers subtraction. There are 6 friends playing at the park. Two of them have to go home early. How many of them are left?

1. Of the 6 friends, 2 have to leave. (6 - 2 =?)
2. Two kids leave, and your students can count the remaining friends.
3. There are 4 kids remaining.

The second subtraction problem is 10 - 3.

1. There are 10 balloons.
2. Boddle takes 3 of the balloons away, asking your students how many are left.
3. There are 7 balloons remaining.
4. So, 10 - 3 = 7.

Want more practice?