Learning how to add and subtract 10 mentally, without counting, is a first grade Common Core math skill: 1.NBT.5. 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 counting from 1 to 100 using 1’s and 10’s, starting from any number. They should also be able to read, write, and represent objects using numbers between 0 and 20 (K.CC.1-3).

**Future Learnings**

Later on, understanding place values will enable your students to skip-count within 1000 (counting by 5’s, 10’s, and 100’s). They will also be able to read and write numbers by using “base ten numerals, number names, and expanded form” (2.NBT.1-3).

**Common Core Standard: 1.NBT.5 - Given a two-digit number, mentally and without counting, find 10 more or 10 less than a number**

Students who understand this principle can:

- Using physical tools, hundred charts, and number lines, find 10 more and 10 less if any two-digit number.
- Without using a tool for help, mentally add 10 to any two-digit number
- Without using a tool for help, mentally subtract 10 from any two-digit number.
- Explain why the digit in the tens place digit changes while the ones place does not change.

**2 Videos to Help You Teach Common Core Standard: 1.NBT.5**

Below we provide and breakdown two videos to help you teach your students this standard.

**Video 1: Visualizing Adding and Subtracting 10**

This video gives students two methods to add or subtract by 10, helping them visualize the process through methods they should already be familiar with.

The video opens with a girl trying to add and subtract by 10; however, it is taking her a long time, so she tries to find a quicker method.

The problem is 41 + 10:

- First method uses a 100s chart.
- Then find 41, and count up by 10.
- We land on 51.
- 51 is in the same column, just below 41.

Then she tries subtracting 10 from 78.

- Using the 100s chart, count back 10 from 78.
- We land on 68.
- It is in the same column, right before 78.

From this, your students can conclude that they can use a hundreds chart to quickly add and subtract by 10. Simply find the number you are adding or subtracting from, and look above or below the number in the same column.

Next, the girl looks at tens and ones sticks. The problem is 41 + 10.

- 41 equals 4 tens sticks and 1 ones stick.
- If a ten stick is added, then there are 5 tens (50).
- But the 1 remains the same.
- So, 41 + 10 = 51.

Next, we try the same with 78 - 10.

- At first, there are 7 tens and 8 ones.
- After a ten is removed, there are 6 tens (60).
- The 8, in the ones place, remains the same.
- So, 78 - 10 = 68.

The video ends by explaining what your students will do as they learn to mentally add and subtract 10 through their program.

**Video 2: Sing Along! Subtracting 10 with Chickens and Cats**

This video is a fun song designed to help your students remember how to subtract 10 from a number while sharing a goofy story they are sure to enjoy.

The story is of a farmer who got a cute chicken who liked to subtract. Unfortunately, she could not add. Such is life. However, the chicken, oddly enough, only got happy when subtracting by 10.

The song then commences, singing about subtracting by 10. A brief description follows.

- The chicken was in the backyard with 25 cats stacked on her head.

a. Those are the facts. - 10 rolled away, and she subtracted by 10.

a. Here’s what the chicken sang.

b. “Subtract one 10, and the ones remain.

c. Drop one ten, the ones stay the same.”

d. And repeat.

The video then explains this principle using tens and ones sticks.

- 25 has 2 tens and 5 ones.
- Take away 1 ten, and only 1 ten remains.
- But the ones, in this case 5, stay the same.
- 10 + 5 = 15.
- And so, 25 - 10 = 15.

The video repeats this pattern of song and explanation two more times, covering the numbers 46 and 102.

After showing your students this video, you can point out that the same principle this video shares applies to addition too: “Add one ten, and the ones remain. Just one more ten, the ones stay the same.”

**Want more practice?**

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*Information on standards is gathered from The New Mexico Public Education Department's New Mexico Instructional Scope for Mathematics and the Common Core website.