# Strategies for Adding & Subtracting within 100

Learning different strategies to add and subtract numbers within 100 is a second grade, Common Core math skill: 2.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 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.5 - Fluently add and subtract within 100 using strategies based on place value, properties of operations, and/or the relationship between addition and subtraction

Students who understand this principle can:

1. Successfully apply different strategies to add and subtract within 100.
2. Explain how addition and subtraction are related to solve problems.
3. Explain and understand the different strategies used to add and subtract.
4. Use place value to add and subtract.
5. Create concrete models or drawings, using strategies to add within 100 (strategies = place value, properties of operations, and how addition & subtraction are related).
6. Utilize number lines, base-ten blocks, or items to find unknown numbers.

Video 1: Using Rods and Cubes to Subtract

This video shows students how to subtract using drawings. Though it only covers subtraction problems, the method can be adapted for addition problems as well.

The first problem is 92 - 34 = ?. How the video solves this problem is related below.

1. Start by representing 92 with 10s rods and 1s cubes.
2. Since there are 9 tens, draw 9 rods.
3. Since there are 2 ones, draw 2 cubes.
4. Now, we will subtract (or take away) 34.
5. The video starts by removing 3 rods.
6. Now we must subtract 4, but removing another rod will remove 10.
7. There aren’t 4 ones to take away, only 2.
8. So take 1 ten rod and break into 10 cubes.
9. Then subtract 4 from the cubes.
10. Now count the remaining pieces to solve the problem.
a. So, 92 - 34 = 58.

The second problem is 43 - 12 = ?. The video follows the same pattern as the first problem to solve the equation.

1. Represent 43 with 4 rods and 3 cubes.
2. Subtract 12 by removing 1 rod and 2 cubes.
3. Count the remaining pieces to find the answer.
a. So, 43 - 12 = 31.

Video 2: Finding the Keys to Treasure Chests

This video covers some strategies for adding and subtracting up to 100. Boddle presents 5 treasure chests that need to be unlocked. However, they can only be opened with the correct key, and to select the correct key, one must solve the math problem.

Boddle asks students if they can help her unlock the chests.

1. Chest 1 has the problem 82 + 0.
a. Remember, any number added to 0 is equal to itself.
b. 82 + 0 = 82.
c. So use the key that says 82.
2. Chest 2 has the problem 50 - 10.
a. Since the values in the ones place are both 0’s, we can simply subtract those in the tens place.
b. 5 - 1  = 4; add the 0 after for the ones place.
c. 50 - 10 = 40; so use the key that says 40.
3. Chest 3 has the problem 41 + __ = 67.
a. What do we need to add to 41 to make it 67.
b. Subtract 41 from 67; position the numbers in a column to make it easier.
c. 67 - 41 = 26
d. 41 + 26 = 67
4. Chest 4 has the problem 62 - 8.
a. Align the numbers in a column and subtract ones place first.
b. Here we get 2 - 8, which we cannot do, so we borrow one from the 6.
c. Now we have 12 - 8 = 4.
d. Since we borrowed one from 6, it becomes a 5.
e. Nothing is subtracted from the new 5, so the answer is 54.
5. Chest 5 has the problem 72 + 15.
a. There is only one key left, so we are sure it’ll open the chest.
b. But let’s see if it's true.
c. Place the numbers in columns and add the place values.
d. 2 + 5 = 7 and 7 + 1 = 8
e. So, 72 + 15 = 87.

Boddle congratulates the students on a job well done, hoping to see them next time.

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