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# Multiplying a decimal by a power of 10 | Decimals | Pre-Algebra | Khan Academy

A single postage

stamp costs $0.44. How much would a roll

of 1,000 stamps cost? And there’s really a

couple of ways to do it. And I’ll do it both ways just

to show you they both work. One is kind of a

faster way, but I want to make sure you

understand why it works. And then we’ll verify

that it actually gives us the right answer using maybe

the more traditional way of multiplying decimals. So we’re starting at $0.44. I’ll just write it as 0.44. Well, that’s one stamp. So this is 1 stamp. I’ll write it like

this, 1 stamp. How much would 10 stamps cost? Well, if 1 stamp is

$0.44, then 10 stamps, we could move the decimal

to the right one place. And so it would be–

and now this leading 0 is not that useful. So it would now be $4.4. Or if you want to make it

clear, it would be $4.40. Now, what happens if you

want to have 100 stamps? Well, the same idea

is going to happen. We’re now taking 10

times more, so we’re going to move the decimal

to the right once. So 100 stamps are

going to cost $44. And this should

make sense for you. If 1 stamp is 44/100

of $1, then 100 stamps are going to be 44/100

of $100, or $44. Or you could view

it as we’ve just moved the decimal

over one place. So if we want 1,000 stamps,

we’d move the decimal to the right one more time. Moving the decimal

to the right is equivalent to multiplying by 10. So then it would be $440. And we could add another

trailing zero just to make clear that there’s

no cents over here. So if you wanted to

do it really quickly, you could have

started with $0.44. And you say look, I’m

not multiplying by 10. I’m not multiplying by 100. I’m multiplying by 1,000. So you’re going to have to

put another trailing zero over here. And you would move the decimal

from over here to over here. You’ve essentially

multiplied this times 10 times 10 times 10,

which is 1,000. So then this would become $440. And let’s verify that this works

exactly the same if we multiply it the traditional way, the

way we multiply decimals. So if you have

1,000 times $0.44. So you start over here. 4 times 0 is 0. 4 times 0 is 0. 4 times 0 is 0. 4 times 1 is 4. Or, you could just say,

hey, this was 4 times 1,000. Then we’re going to

go one place over. So we’re going to add a 0. And once again, we’re going

to have 4 times 0 is 0. 4 times 0 is 0. 4 times 0 is 0. 4 times 1 is 4. Or we just did 4 times 1,000. So that is 4,000 if you

don’t include this 0 that we added here ahead

of time because we’re going one place to the left. And then we have nothing left. I haven’t at all thought

about the decimals right now. So far I really just viewed

it as 1,000 times 44. I’ve been ignoring the decimal. So if it was 1,000 times 44,

we would get 0 plus 0 is 0. 0 plus 0 is 0. 0 plus 0 is 0. 4 plus 0 is 4. 4 plus nothing is 4. And if you ignore the decimal,

that makes a lot of sense because 1,000 times 4 is 4,000. And 1,000 times 40

would be 40,000. So you would get 44,000. But this, of

course, is not a 44. This is a 44/100. We have, between the

two numbers, two numbers behind the decimal point. So we need to have

two numbers behind or to the right of the

decimal point in our answer, so one, two right over there. So once again, we get

$440 for the 1,000 stamps.

60 comments

Too easy chap, too easy… 🙂

@ylun96 haha

now i get it

so sad

yolo

Ha gay this is so gay and boring

very helpful

Not helpful 😒 the first way!!!

That is not how u do power of ten .example: like 10 to the 2 power. Another example :so we r going to start out with 37.2:10 to the 0 power is 37.2,10 to the 1st power is 372, 10 to the 2 power is 3,720, 10 to the 3rd power is 37,200 that is how u do powers of ten. But here is the thing u have to know the decimal goes to the right each time.

thank you it helped alot

get it know very helpful khan academy time to SUBSCRIBE TO U!!!!!!!!! AYAYAYAYAYAYAYAYYA

thank you dude.

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Sal dude your amazing what would i do without you man #allhailsall

Thank you man hopefully i will pass my maths test tomorrow now!

thank you very much…

it helps me in my assign ment

Thank you !!! nlw i hope i get a high grade on the math exam tomorrow

ha nice yah saved my life but how did you know that

Not sure if it's just me but reading these as apposed to seeing an listening is night and day when it comes to shortcuts. My book explained this in shorter terms but made it seem 10 times more complicated

hayspookay

Omg thankyou so much

tangina niyo bobo peke yan

it means wow beatiful

Can you tell me the program you use

Awesome

i feel like this helped me

This sucks it should be banned from YouTube

Hi dood I love your video so entertaining

thanks so much u helped a toooooooooon

khan top laner of lz

helped me a lot

you should add music to make it better

Not helpful 😡😠😤😠😠

👏

i dont like math

just joking. you are awesome

I REALLY HATE MATH

JK YOU ARE AMAZING

Thank you! Helped alot

Thank you

YOUR RONG!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

it helped a lot

i don't get it

Unhelpful

how

0hhhhhh. Now i get it.

Too fast

Not helpful too i agree

this really helped a lot thank you

I still don't get it

I dont understand 🤔🤔🤔🤔🤔🤔😭😭😭😭

your not explaining how to do it ;-;

does the decimal point stay the same

I hate this.

Thanks hopefully I pass my test

makes sense

My teacher told us to watch these

2019?

205

why do u speed up your videos