Basic Math Multiplying Decimals

Welcome back to Educator.com; this lesson is on multiplying decimals.0003

When you multiply decimals together, it is very different than when you are adding and subtracting decimals.0007

The rules are very different; try not to get confused between the two.0015

Remember when you add and subtract the decimals, you have to line up the decimal point.0020

Then you add and subtract.0025

Then you bring the decimal point straight down into the answer.0026

When you multiply decimals, you don't worry about the decimal point at all.0030

If I am going to multiply two numbers, let's say 1.1 and 6.0038

I don't have to line up the decimal point; make sure you don't do that.0048

All you have to do is multiply the numbers without having any consideration for the decimal point.0051

I am just going to ignore it; I am going to multiply this.0058

It is going to be 6; and then 6; 66.0061

What you do is you count the total number of decimal places from the numbers you multiplied.0066

From these two numbers, this one and this one,0071

you are going to count to see how many numbers are behind the decimal point.0074

Here I have one number.0080

Here I have none because the decimal point is behind the 6.0082

From these two numbers, from 1.1 and 6, I only have one number behind the decimal point.0087

I go to my answer; I place one number behind the decimal point.0098

The last number is 6; I am going to put the decimal point right there, 6.6.0103

Let's do a few examples, 0.2 times 0.6.0111

Again I am going to multiply the numbers without considering the decimal points.0117

6 times 2 is 12.0126

I don't have to multiply those 0s together; it is just 12.0129

From this number, from these two numbers, the two numbers that I multiplied,0134

I am going to see how many numbers I have behind the decimal point.0138

From this number, I have one; from this number, I have another one.0143

I have two total; I go to my answer.0147

I am going to place two numbers behind the decimal point.0151

It is going to become 0.12 or 0.12 or 0.12.0155

I can put a 0 up here too.0161

This is the whole number; we don't have any whole numbers; it is just 0.0163

Another example, if Susan works 25.5 hours per week and she earns0170

9 dollars and 40 cents an hour, how much does she earn in a week?0177

This is how many hours she works in a week.0184

This is how much she earns per hour.0186

To figure out how much she earns in a whole week,0189

I have to multiply how many hours she worked with how much she makes per hour.0191

It is going to be 25.5 times 9 dollars and 40 cents.0200

Again when I multiply these numbers, I am just going to line up the numbers.0210

I don't care about the decimal point.0215

25.5 times 9.40; you are just lining up the numbers.0218

Let's multiply the 0; 0 times 0 is going to be all 0s.0232

I am just going to move on to the next number.0237

4 times 5 is 20; this is 22; 8, 9, 10.0238

9 times 5 is 45; 45... that is 49; 18... that is 22.0248

These are just 0s here; it is 0, 0, 7, 9, 3, and 2.0266

Here is my answer when I multiply these two numbers together.0276

Now I have to look at my decimal point.0280

The first number, I look at these two numbers, the two numbers that I multiplied.0284

I have one number here behind the decimal point and I have two numbers here.0288

How many numbers do I have total?--I have three.0295

I go to my answer; I count three numbers.0300

Make sure that I have three numbers behind my decimal point.0305

It is going to be 237.7.0311

I am dealing with money here because I am trying to figure out how much she earns in a week.0317

This is going to be in money; I am going to have a dollar sign.0323

This becomes 239 dollars and 70 cents.0330

This is how much she is going to earn in a week.0338

The next example, I have 0.21 times 2.1.0346

0.21 or I can read this as 21 hundredths because I have two numbers.0356

This is tenths; this is hundredths.0363

This would be 21 hundredths times 2.1.0365

Again I am not going to line up the decimal point; 2.1 or 2 and 1 tenths.0372

When I multiply this out, ignore the decimal point.0383

2 times 1; write that here; that is 2; 2 times 2 is 4.0389

You don't have to look at this number.0395

If you want, you can just write that down.0397

1 times 0 is 0; that goes there; then I add these down.0399

1 plus nothing is 1; 2 plus 2 is 4; this is 4.0406

Now I look at my numbers; how many numbers do I have behind decimal points?0413

Here I have two; here I have another one; I have three total.0421

I am going to go to my answer; I am going to count one, two, three.0428

My answer, I have to make sure that there is going to be three numbers,0436

the same number of numbers behind this decimal point.0439

It is going to be 0.441 or 0.441.0442

You can read this as 441 thousandths because I have three numbers and this is the thousandths place.0451

This is my answer when I multiply 0.21 times 2.1.0460

The fourth example, I am going to multiply these two numbers, 4.08 and 1.35.0468

4 and 8 hundredths... again when I multiply these decimals together, I am just going to ignore my decimal points.0479

I am just going to line up the numbers just like I do when I multiply whole numbers.0488

This just happens to line up because there is the same number of numbers.0495

Multiply this out; 8 times 5 is 40; 0, 4; this is 20.0502

This is 24, 0, 2; this is 12 here; this is 8, 0, and 4.0512

I add them down; 0; this is 8; this is 10.0528

2, 4, 5; this is also 5.0539

From here, after I multiply my two numbers, I am going to look at the actual numbers that I multiplied.0545

I am going to count how many numbers I have behind decimal points.0554

For this one, I have two numbers behind the decimal point.0558

Here I also have two; total behind the decimal points, I have four numbers.0562

You look at just these two numbers that you multiplied together.0570

I have four numbers total behind decimal points; I go to my answer.0574

I make sure that there is four numbers behind the decimal point.0579

That is going to help me place the decimal point.0583

One, two, three, four; there is four numbers; place the decimal point right there.0587

Since there is four numbers behind decimal points here,0595

there has to be four numbers behind the decimal point in the answer.0598

It is going to be 5.5080.0601

Or this number, it is a 0 at the end of a number behind the decimal point.0605

I can just drop it if I want.0609

Or you can leave it; it doesn't matter.0611

It could be 5 and 508 thousandths.0613

Either way, this can be the answer or this can be the answer.0620

That is it for this lesson on multiplying decimals; thank you for watching Educator.com.0626