Subtraction with decimal numbers with 3 decimal places
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General
Students learn to subtract decimal numbers with 3 decimal places. They also learn to solve story problems with decimal numbers.
Common core standard(s)
6.NS.B.3
Relevance
Discuss with students that it is important to be able to subtract decimal numbers with three decimal places, so you can determine how many seconds faster you were in a sprint.
Introduction
Students start the lesson by practicing completing a decimal place value chart. Students are given a number and must drag the numbers to where they belong on the decimal place value chart. Erase the grey boxes to show the correct answer.
Development
Using the number line, show students how to subtract decimal numbers with 3 decimal places. Show students that you leave the first number (the minuend,4.756) whole and that you split the second number (the subtrahend, 1.513) into 1 and .513. Then using the number line, start by taking away 1, and then take away the 0.513. Explain to students that they also have a second option for subtraction. They can choose to split both the first(minuend) and second(subtrahend) numbers into whole numbers and numbers after the decimal point. In the subtraction problem 4.756 - 1.513 it would turn into 4 - 1 and .756 - .513. Solve both of these subtraction problems and add the differences together. Check that students are able to apply their understanding by solving the subtraction problem with 3 decimal places. Emphasize that students may choose which method to use to subtract the decimal numbers, but that you cannot use the second method if the numbers in the decimal places of the first number are less than those of the second number. Using the story problem with the lion, explain to students how to solve a decimal number story problem. Go through the steps and then ask students to solve the next story problem individually or in pairs. Have them name the steps they take to solve the decimal story problem.
Check that students are able to subtract decimal numbers with 3 decimal places by asking the following questions:
- Which methods can you use to subtract decimal numbers with 3 decimal places?
- Which method do you like best?
- How do you solve a story problem?
- What are the steps of solving a story problem?
Guided practice
Students are given subtraction problems with decimal numbers that have 3 decimal places. They are asked to either select the answer from a set of multiple choice options or to fill in the correct answer. Students are also asked to solve decimal story problems. Remind students to use the steps of solving a story problem.
Closing
Discuss the learning goal with students and that it is important to learn subtraction with decimal numbers with 3 decimal places, so you can figure out the difference in speed between two sprints. Check that students are able to subtract decimal numbers with 3 decimal places by having them solve a set of problems. Then ask students to solve a story problem and name the steps required. Have students form pairs and come up with subtraction problems for their partner to solve that have 1, 2, and 3 numbers before the decimal point and that all have 3 decimal places.
Teaching tips
Students who have difficulty with subtracting decimal numbers with 3 decimal places can be supported by making use of the number line. Have students first place decimal numbers with 3 decimal places on the number line and ask them to subtract 0.001 from the number. Where do they go on the number line? Repeat this particularly when crossing tens, so from 1.010 to 1.009, or 1.100 to 1.099.
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