Multiply decimals by whole numbers – 5th grade

To multiply decimals by whole numbers for 5th graders, we can break it down into simple steps with examples. Here’s how we can explain it:

Steps to Multiply Decimals by Whole Numbers:

1. Ignore the Decimal Point Initially: Treat the decimal number as a whole number. Multiply it by the whole number just as we would with regular whole numbers.
2. Multiply: Perform the multiplication as if there were no decimal point.
3. Count Decimal Places: After multiplying, count the number of decimal places in the original decimal number. This tells where to place the decimal point in the final product.
4. Place the Decimal Point: Place the decimal point in the product so that it has the same number of decimal places as the original decimal number.
5. Write the Final Answer: The result get after placing the decimal point is the final answer.

Example 1: Multiply 3.2 by 4

1. Ignore the decimal point: Think of 3.2 as 32.
2. Multiply: 32 × 4 = 128.
3. Count Decimal Places: 3.2 has 1 decimal place.
4. Place the Decimal Point: Move the decimal point in 128 one place to the left to get 12.8.
5. Final Answer: 3.2 × 4 = 12.8.

Example 2: Multiply 5.67 by 3

1. Ignore the decimal point: Think of 5.67 as 567.
2. Multiply: 567 × 3 = 1701.
3. Count Decimal Places: 5.67 has 2 decimal places.
4. Place the Decimal Point: Move the decimal point in 1701 two places to the left to get 17.01.
5. Final Answer: 5.67 × 3 = 17.01.

Example with a Diagram

Let’s use 2.5 × 6:

1. Ignore the decimal point: Think of 2.5 as 25.
2. Multiply: 25 × 6 = 150.
3. Count Decimal Places: 2.5 has 1 decimal place.
4. Place the Decimal Point: Move the decimal point in 150 one place to the left to get 15.0.
5. Final Answer: 2.5 × 6 = 15.0.

To help visualize:

  2.5
×   6
------
  150   (Step 2)

Since there’s 1 decimal place in 2.5, we place the decimal one digit from the right in the result, making it 15.0.

This method helps 5th graders understand the process of multiplying decimals by whole numbers systematically.