LESSON: Multiplying Decimals
READ: Multiplying Decimals by Whole Numbers
Multiplying Decimals by Whole Numbers
Multiplication is a short-cut for repeated addition. We think about multiplication and we think about groups of numbers.
Example
4 3
12
With this example, we are saying that we have four groups of three that we are counting or we have three groups of four. It doesn’t matter which way we say it, because we still end up with twelve.
When we multiply decimals and whole numbers, we need to think of it as groups too.
Example
2(.25) = _____
With this example, we are multiplying two times twenty-five hundredths. Remember that when we see a number outside of the parentheses that the operation is multiplication.
We can think of this as two groups of twenty-five hundredths. Let’s look at what a picture of this would look like.
Our answer is .50.
This is one way to multiply decimals and whole numbers; however we can’t always use a drawing. It just isn’t practical.
How can we multiply decimals and whole numbers without using a drawing?
We can multiply a decimal and a whole number just like we would two whole numbers. First, we ignore the decimal point and just multiply. Then, we put the decimal point in the product by counting the correct number of places.
Example
4(1.25) = _____
Let’s start by multiplying just like we would if this was two whole numbers. We take the four and multiply it with each digit in the top number.
Our work isn’t finished yet. We need to add the decimal point into the product. There were two decimal places in our original problem. There needs to be two decimal places in our product.
This is our final answer.