Subtraction with decimal numbers with 1,2, and 3 decimal places
Subtraction with decimal numbers with 1,2, and 3 decimal places

Subtraction with decimal numbers with 1,2, and 3 decimal places

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General

The students learn to subtract decimal numbers with 1, 2, and 3 decimal places. They also learn how to determine which numbers are important within a story problem.

Common core standard(s)

6.NS.B.3

Relevance

It is important that students learn to subtract decimal numbers with 1, 2, and 3 decimal places so you can determine how much weight is left.

Introduction

Students practice subtraction problems with 1, 2, or 3 decimal places. Each subtraction problem has the same number of decimal places.

Development

Using the number line, explain to students how you subtract decimal numbers with 1, 2, and 3 decimal numbers. Tell students that there are 2 methods they can use to subtract. The first method is to split the subtrahend and to take it away from the first number in parts. Show that you leave the first number (92.86) whole and that you split the subtrahend (61.425) into two parts 61 and 0.425. Use the number line to take away the 61 and then take away the 0.425. Then show them the second method. In the second method split both the minuend and subtrahend into numbers before the decimal point and after the decimal point. With the problem 92.86 - 61.425 the subtraction problem becomes 92 - 61 and 0.86 - 0.245. Solve both problems and add the two answers together. Remind students that they can't use this second method if the numbers after the decimal point of the subtrahend are greater than the numbers after the decimal point of the first number. Give students a few subtraction problems to check that students are able to subtract decimal numbers with 1, 2, and 3 decimal places. Ask them to explain their strategy and remind students that they can choose which method they feel works better.
Then discuss story problems with decimal numbers. Remind students of the steps to solve a story problem. First they need to determine what kind of math problem it is, then to determine which numbers are important, then to say or write the problem and finally to solve. Solve a story problem as a class and then ask students to solve the next story problem individually or in pairs.

Check that students understand how to subtract decimal numbers with 1, 2, and 3 decimal places by asking the following questions:
- Which methods can you use to subtract decimal numbers with 1, 2, and 3 decimal places?
- Which method do you like?
- How do you solve a story problem?
- What are the steps to solve a story problem?

Guided practice

Students are given subtraction problems with decimal numbers with 1, 2, and 3 decimal places. Students are asked to select the correct answer from multiple choice options as well as filling in the correct answer. Students are also asked to solve story problems.

Closing

Check that students are able to explain which method they use when subtracting decimal numbers with 1, 2, and 3 decimal points. Ask students to explain how to solve a story problem. Use the subtraction problems on the interactive whiteboard to check that students are able to subtract the decimal numbers with 1, 2, and 3 decimal places. Emphasize that students may choose which method to use. Students can also be challenged with the problems under the "extra" box. These subtraction problems have multiple regroupings within one problem.
For the last problem- split classes into groups of four. Each students selects an animal and then they must solve each step of the subtraction problems and determine which of the animals gets closest to the final number (459.7). Only one animal ends exactly on 459.7 (it's the goat).

Teaching tips

Students who have difficulty with this learning goal can practice placing decimal numbers on the number line. Have them then practice subtracting by 0.001, by 0.010, and by 0.100. Also work with students to practice the steps between 1 to 0.9, or 10 to 9.9. Then have students practice with crossing with 2 decimal places or 3 decimal places.

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