Lesson 10: Multiply Decimals by Whole Numbers

Property

To multiply a decimal by a whole number:

1. Multiply the numbers as if they were whole numbers, ignoring the decimal point.
2. Count the number of decimal places in the decimal factor.
3. Place the decimal point in the product so it has the same number of decimal places as the decimal factor.

Examples

Property

To multiply a decimal by a whole number, first multiply the numbers as if they were whole numbers. Then, place the decimal point in the product so that it has the same number of decimal places as the decimal factor.

Examples

• To solve $0.3 \times 4$, first calculate the whole number fact $3 \times 4 = 12$. Since $0.3$ has one decimal place, the product will also have one decimal place: $1.2$.
• To solve $1.25 \times 3$, first calculate the whole number fact $125 \times 3 = 375$. Since $1.25$ has two decimal places, the product will also have two decimal places: $3.75$.

Explanation

This skill helps you use your knowledge of whole number multiplication to solve decimal problems. By temporarily ignoring the decimal point, you can calculate the digits of the answer using familiar facts. Then, you simply place the decimal point correctly in the final product. The number of decimal places in your answer must match the number of decimal places in the decimal factor you started with.