Adding using standard algorithm with decimal numbers to the thousandths place

General

Students learn to add decimal numbers to the thousandths place using the standard algorithm, with and without regrouping.

Common core standard(s)

CCSS.Math.Content.6.NS.B.3

Relevance

Discuss with students that it is useful to be able to use the standard algorithm on decimal numbers, so you can determine decimal totals. You can use this, for example, when shopping to determine the total.

Introduction

Ask students to listen to the amount spoken aloud and have students drag the numbers to create that money amount.

Development

Remind students that decimal numbers are built up of whole numbers, a decimal point, and numbers in the decimal places. The places after the decimal point are called tenths, hundredths, and thousandths. When adding using the standard algorithm you write the numbers one below the other and then add from right to left. Refresh what all the letters in the decimal place value chart stand for and then show students step by step how to add decimal numbers using the standard algorithm. Show students that you start on the right. You add the numbers in the tenths column together. If the number is greater than ten, remind students that the tens go high, the ones stay low, and that you write the tens in the little box above the column to the left- this is regrouping (you are adding those tens to the ones column in this case). Then add the numbers in the ones column- and don't forget to add the 1 from the little box too! After adding all of the numbers in the ones column together (top, bottom, and little box) you have determined the total. The decimal point never moves or changes places. Repeat these steps with addition problems with hundredths, thousandths, and multiple addends. Ask students to practice addition problems with tenths, hundredths, thousandths and multiple addends after each segment is taught.

Check that students are able to add decimal numbers to the thousandths place using the standard algorithm by asking the following questions:
- Why is it useful to be able to use the standard algorithm?
- What happens with the decimal point?
- What do the letters: H T O t h th stand for?
- Where do you start when solving using the standard algorithm?
- Solve these problems using the standard algorithm: 1.2 + 4.9 = ?, 9.87 + 4.35= ?. 27.95 + 34.24 = ?

Guided practice

Students practice adding decimal numbers with 1, 2, and 3 decimal places. They must also practice adding multiple addends together.

Closing

Ask students to form pairs and to use the images (and prices) on the interactive whiteboard to create and solve some addition problems. Remind students to solve from right to left. Remind students why it is useful to use the standard algorithm. Students can be challenged to create addition problems with multiple addends.

Teaching tips

Students who have difficulty with the standard algorithm can be supported by a refresher on the decimal place value chart as well as the concept of regrouping. This can be shown by the use of manipulatives or by writing out each addition problem of the place value numbers (similar to partial sums).