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Lesson 3: Multiplying decimals

# Multiply decimals: FAQ

## Why is it important to know how to estimate when multiplying decimals?

Estimating can be helpful for a few reasons. First, it can give us a rough idea of what the product should be, which can help catch mistakes. For example, if we estimate that $2×4.8$ should be around $10$, but we end up with a product of $100$, we'll know that something went wrong. Additionally, sometimes we might not need an exact answer, just a ballpark figure. In those cases, estimating might be all we need to do.
Try it yourself with these exercises:

## How do I use a grid or area model to multiply decimals?

First, divide the grid into sections based on the numbers you're multiplying. Then, label the sections with the correct decimal values. Finally, find the area of each section and add them together to get the product.
Try it yourself with these exercises:

## Why is it helpful to understand how to multiply whole numbers by $0.1$‍  and $0.01$‍ ?

Multiplying by $0.1$ and $0.01$ are relatively simple cases of decimal multiplication, so they can help build confidence and understanding before moving on to more complicated examples.
They provide a clear illustration of how the place value of the digits in a number changes when you multiply by a decimal. For example, when you multiply a whole number by $0.1$, all the digits shift one place to the right (e.g. $5$ becomes $0.5$, $12$ becomes $1.2$, etc.).
Multiplying by $0.1$ and $0.01$ are both common calculations in various contexts (e.g. finding $10\mathrm{%}$ or $1\mathrm{%}$ of a number), so it can be useful to know how to do them quickly and accurately.
Try it yourself with this exercise:

## How do we multiply decimals by decimals?

When we multiply decimals by decimals, we multiply the two numbers as if they were whole numbers. We then count the total number of digits after the decimal point in both numbers, and place the decimal point in the product so that there are the same number of digits after the decimal point.
Try it yourself with these exercises:

## Why do I need to understand the different ways to multiply decimals?

Different methods work better for different people. By understanding different ways to multiply decimals, you can choose the one that makes the most sense to you.

## Want to join the conversation?

• One of the questions in this section was
24.78 x.05
You’ve said in the past that this is the same as 24 x .05 plus .78 x .05
24 x .05 = 24/1 x 5/100= which is 120/100, or 1 20/100,
.78 x .05 = 78/100 x 5/100 = 390/1000 or 39/100
1 20/100 + 39/100 = 1 59/100 or 1.59, which is wrong. The answer is 1.2390. Where did I go wrong? Should we not reduce fractions like 390/1000?