Learn on PengiBig Ideas Math, Course 1Chapter 2: Fractions and Decimals

Lesson 4: Adding and Subtracting Decimals

In this Grade 6 lesson from Big Ideas Math, Course 1, students learn how to add and subtract decimals by lining up decimal points and inserting zeros to match decimal places. The lesson begins with base ten block models and place value charts to build understanding before moving to the standard vertical algorithm. Students also apply these skills to real-life problems involving money and perimeter calculations.

Section 1

Add and Subtract Decimals

Property

To add or subtract decimals, we line up the decimal points. By lining up the decimal points this way, we can add or subtract the corresponding place values. The steps are:

Determine the sign of the sum or difference.
Write the numbers so the decimal points line up vertically.
Use zeros as placeholders, as needed.
Add or subtract the numbers as if they were whole numbers. Then place the decimal point in the answer under the decimal points.

Examples

Add $19.45 + 7.8$ . We line up the decimal points: $19.45 + 7.80 = 27.25$ .

Subtract $8.15 - 12$ . The result will be negative. We subtract the smaller value from the larger: $12.00 - 8.15 = 3.85$ . So, the answer is $-3.85$ .

Section 2

Calculating Totals and Change in Financial Problems

Property

To solve real-world problems involving decimals, translate the words into a mathematical expression.
Use addition to find a total or combine quantities.
Use subtraction to find a difference, how much is left, or how much more is needed.

Examples

Lesson overview

Expand to review the lesson summary and core properties.

Expand

Section 1

Add and Subtract Decimals

Property

To add or subtract decimals, we line up the decimal points. By lining up the decimal points this way, we can add or subtract the corresponding place values. The steps are:

Determine the sign of the sum or difference.
Write the numbers so the decimal points line up vertically.
Use zeros as placeholders, as needed.
Add or subtract the numbers as if they were whole numbers. Then place the decimal point in the answer under the decimal points.

Examples

Add $19.45 + 7.8$ . We line up the decimal points: $19.45 + 7.80 = 27.25$ .

Subtract $8.15 - 12$ . The result will be negative. We subtract the smaller value from the larger: $12.00 - 8.15 = 3.85$ . So, the answer is $-3.85$ .

Section 2

Calculating Totals and Change in Financial Problems

Property

Examples

Overview

Lesson overview

Review the lesson summary and core properties without leaving the current page.

Overview

Section 1

Add and Subtract Decimals

Property

To add or subtract decimals, we line up the decimal points. By lining up the decimal points this way, we can add or subtract the corresponding place values. The steps are:

Determine the sign of the sum or difference.
Write the numbers so the decimal points line up vertically.
Use zeros as placeholders, as needed.
Add or subtract the numbers as if they were whole numbers. Then place the decimal point in the answer under the decimal points.

Examples

Add $19.45 + 7.8$ . We line up the decimal points: $19.45 + 7.80 = 27.25$ .

Subtract $8.15 - 12$ . The result will be negative. We subtract the smaller value from the larger: $12.00 - 8.15 = 3.85$ . So, the answer is $-3.85$ .

Section 2

Calculating Totals and Change in Financial Problems