Subtracting numbers to 100 using tens

General

Students learn to subtract numbers to 100 using tens.

Common core standard(s)

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

Relevance

Discuss with students that it is important to be able to subtract numbers to 100, so you can determine how much you have left.

Introduction

Show a few subtraction problems on the interactive whiteboard that use the zero rule. Ask students which answer belongs to each subtraction problem and drag the answers to the correct place.

Development

Start by explaining that tens are a group of ten and always end on a 0. The learning goal is shown using visual support, using abstract numbers, and as story problems. You can navigate between these methods using the blue menu on the bottom right to select the method best suited to your classroom. Next explain that you can subtract by counting amounts. You start by counting how many objects there are in total, this is the first number of your subtraction problem (the minuend). Next you count how many objects are taken away, this becomes the second number of your subtraction problem (the subtrahend). When you take the second number from the first you are left with the difference. Show this on the interactive whiteboard using the images of lemonade and ice cubes, discussing the different subtraction problems. Check that students are able to do this by asking them to subtract the chicken feet and the blocks. Ask them to say or write the subtraction problem as well. Next tell students that you can subtract different problems in different ways. You can use a number line, you can see how many more need to be added to the subtrahend to equal the minuend. Emphasize that you must carefully look at the numbers in the subtraction problem to determine the best way to subtract the given numbers. Ask students to solve a few subtraction problems and discuss their strategy. Make sure that students are aware that all strategy is valid- it has to do with what makes the subtraction problem easier for them to solve. Finally discuss with students how to solve story problems. Have students determine what kind of math problem is being told, which numbers are required for the math problem, to say or write the problem, and finally to solve the math problem. Do one story problem as a class, and then ask students to solve some story problems individually or in pairs.

Check that students are able to subtract numbers to 100 by asking the following questions:
- What steps do you take to solve a story problem?
- How do you solve 78 - 30 and 40 - 30?

Guided practice

Students are first given subtraction problems with visual support, then are given abstract subtraction problems, and are then given subtraction story problems.

Closing

Discuss with students that it is important to be able to subtract numbers to 100 because you then are able to determine how much you have left with larger quantities. Show a few images on the interactive whiteboard and ask students how they would solve the problems. Then ask students to form groups. Each student solves the subtraction problems shown in the purple circle. Which student is first at finding the difference? Ask students to compare their answers. Did they all solve in the same way and reach the same difference?