# Mentally Add & Subtract 10 or 100

Learning to confidently add and subtract 10 or 100 from a number between 100-900 is a second grade, Common Core math skill: 2.NBT.8. 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 first grade skill of counting up to 120 starting from any number below 120 (1.NBT.1), this skill helps them understand greater or less than values. The second grade skill is also closely linked to the first grade skill of understanding place values (ones and tens) in two-digit numbers (1.NBT.2).

Future Learnings

Comparing large numbers as greater than, less than, and equal to will help your students understand future concepts in third grade. In third grade, your students will learn how to interpret the products of whole numbers (i.e. 8 x 3 is the same as 8 groups of 3 objects each) (3.OA.1). They will also learn to use multiplication and division within 100 while solving word problems in situations that involve “equal groups, arrays, and measurement quantities” (3.OA.3).

Common Core Standard: 2.NBT.8 - Mentally add and subtract 10 or 100 to a given number between 100-900

Students who understand this principle can:

1. Mentally add 10 or 100 from any number between 100-900, using place value.
2. Mentally subtract 10 or 100 from any number between 100-900, using place value.

Video 1: What’s 100 Points More or Less?

The video begins by introducing a word problem: “Alice’s teacher gives her 10 points for every A she earns on a test. Alice has 110 points already. [That’s a lot of tests!] How many points will she have if she earns 100 more?”

1. 110 + 100 = ?
2. You can use place value to help solve this.
3. When you add 100, the digit in the hundreds place goes up by 1 and the rest stay the same.
4. So, 100 + 100 = 200.
5. Add the remaining digits to get 210.

If Alice receives 100 more points, she will have 210 in total.

Now, assume the question asked if Alice lost 100 points. What’s 110 - 100?

1. Since we’re subtracting 100, only the digit in the hundreds place is affected.
2. If you take 100 away from 100, then there are no more hundreds!
3. We are left with only 10.

So if Alice lost 100 points, she’d only have 10 left.

Video 2: Exercising while Subtracting!

This video offers students a chance to move and exercise while learning to mentally subtract 10 from a number. The concept can easily be adapted for addition as well!

The video starts by explaining what the students will do and the rules of the game. Students will do an exercise depending on the number inside the box. For instance, 90 - 10 = 80; 8 is in the box, so they would do the exercise 8 times, not 80.

Below is the subtraction problems and the exercise related to them:

1. What’s 10 less than 280?
a. It’s 270!
b. 7 is in the box.
c. Students will do 7 lunges.
2. What’s 10 less than 542?
a. It’s 532!
b. 3 is in the box.
c. Students will do 3 toe touches.
3. What’s 10 less than 777?
a. It’s 767!
b. 6 is in the box.
c. Students will do 6 knee grabs.
4. What’s 10 less than 698?
a. It’s 688.
b. 8 is in the box.
c. Students will do 8 arm rolls.

That’s it! Hopefully this video helps inspire some fun ideas for classroom learning!

Want more practice?