Addition to 30 using simplification

General

Students learn that you can simplify the addition problem to tens and ones. If you know the total of the addition problem without the tens, you can calculate the addition problem with tens quickly.

Standards

2.OA.A.1
2.NBT.B.5

Introduction

Start with some counting to 30 exercises. Have students count and then explain how they counted, for example with skip counting in 5s or 10s. Next, compose numbers to 30 to a tens and ones number. An example is 23 to 20 and 3.

Instruction

Show students a selection of addition problems that have been simplified into tens and ones. Ask students if they notice anything in particular. They should note that the difference between the two totals is 10. These addition problems belong together. Tell students that you can solve an addition problem with tens faster if you first calculate the ones without the tens. You can use it as a step to your actual total. If you know 4+2 = 6, then you also know 14+2=16, or 24+2=26. Practice with MAB blocks or other manipulatives. Give students a set of 10 blocks as well as loose blocks. Ask students to set out blocks to the problems you mention. An example would be 6 and 3 blocks, ask how many they have. Then add the tens block, so you get 16+3. What is the total now? Emphasize that they don't need to count all of the blocks, because they can use the first step. You add 10 to an addend, so you add 10 to the total. Demonstrate this on the board by dragging the blocks. Check to see that students are also able to solve this without dragging blocks by doing the problem with gum. First 5+3, and then 15 +3, and then 25 +3. Practice a few addition problems with the class without visual support. They can imagine using the tens-block.

Check that students understand addition to 30 using simplification by asking the following questions:
- 6+2= 8, 26+2 = ?
- 4+3= 7, 4+23 is 27, how do you know that?
- 4+5=9. Which addition problem can you now easily solve?

Quiz

Students first solve an addition problem in which the first step problem is shown. Then they are asked to solve the first step as well as the actual problem. They are then challenged to make tens in their addition.

Closing

Repeat why it is useful to learn how to simplify addition. It helps make calculations easier and faster. If you know the addition problem without tens, you can also do the problem with tens. Check that students have understood by doing a few problems in which they have visual support, and a few where they don't have visual support.

Teaching tips

For students who have difficulty, they can be supported by using MAB blocks or other manipulatives like money or the rekenrek. Give them 1 dollar and 1 dollar. They know that the problem is 1+1 = 2. If you add a 10 dollar bill, it becomes 11+1= 12 dollars. Repeat with other amounts. Emphasize that they shouldn't recalculate but should just add 10 to the previous total.