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## Arithmetic

### Unit 17: Lesson 2

Multiplying and dividing decimals by 10, 100, and 1,000# Multiplying and dividing decimals by 10

CCSS.Math:

Learn to multiply and divide decimals by 10 using fraction and place value understanding.

## Want to join the conversation?

- When dividing 3 by .6 is this the method that I can use?

(3 x 10 ) divided by (.6 x 10)

30/6 simplified 15/2

And that’s how you do it or did I miss a step or didn’t do something correctly I need to under stand better the concept of this method(17 votes)- That is a very good way of doing it, move the decimal in the denominator until you get a whole number then move numerator the same amount (multiply by 10 is same as moving decimal).(12 votes)

- Is Mr. Sal Khan multiplying or dividing 67.5 by one tenth?(7 votes)
- He is multiplying and dividing by one 10, so x 10. For example, 42 x 10 you just have to add a extra 0, the way you know how many 0's is by looking behind the number putting the 0's behind the other number.

Hope this helps(7 votes)

- how do i do something like this 2.34 multiplied by 4000 and the same with dividing(7 votes)
- Well you could do it like this: take the 2.34 and move the digits over
*three*spaces (using the same method you would with a 10) to the left to make 2340. Then multiply that by 4 (from the 4000) to get 9360.

And as for division, just do the same thing but slightly reversed. Move 2.34 three spaces to*right*to get 0.00234. Then divide that by 4 (smh) to get 0.000585. And if you're not sure, just check your calculator.

Hope this clears thing up a little for ya :D Peace(3 votes)

- I still don't get it. Can anyone tell me an easier way?(7 votes)
- An easier way is to think about it by look at the number like for example, 23 x 10. When you look at it you just think of it of adding an extra 0. So it would be 230, with a hundred it would be adding two extra 0's and so on.

Hope this help(3 votes)

- i watched it a few times an i am getting them all right(5 votes)
- on the last question you accidentally put a times symbol instead of a dividing symbol.(4 votes)
- it helped but the second equation did not(4 votes)
- Can I use the calculater to calculate hard eqations?(2 votes)
- If you do the arithmetic by hand, then it gives you good practice so that you'll be faster and more accurate, as you won't have a calculator beside you every time. Plus, it's good to know what a calculator is doing. However, if you really truly know how to do the math and have mastered it, there's no sense in doing long, tedious problems by hand all the time. Then you can use a calculator.

Typically, you'll see more calculators the higher up you go in math, as it's expected that everyone knows basic arithmetic, so that isn't tested.(5 votes)

- 3:54I thought we were multiplying, not dividing.(2 votes)
- Actually, we're doing both. That's why the title of the video is Multiplying and dividing decimals by 10(4 votes)

## Video transcript

- [Instructor] We've already learned that when we multiply by 10, let's say we took the number 53 and we were to multiply it by 10, it has the effect of
shifting all the digits once place to the left. So, this should be a review for you. But this was going to be 530. And we could see that what
used to be in the tens place has been shifted to the
left, to the hundreds place. And what used to be in the ones place has been shifted to the
left to the tens place now. And we saw, if you divide by 10, you have the opposite effect. And so, let's say if I had 120, I could say, let's divide by 10, I could also say this is the
same thing as 120 times 1/10. What is going to happen there? Well, in this situation, all the digits are going to
shift one place to the right. So, what used to be in the tens place will now be in the ones place; what used to be in the hundreds place will now be in the tens place. So, this is just going to be equal to 12. So, that was all review. But now were going to
extend this a little bit by thinking about things
that have place values representing less than a
one, I guess you could say, or we're gonna deal with decimals. So, just to get ourselves warmed up, let's see if we could
figure out what 3.015 times 10 is. Pause this video, and see
if you can figure that out. Well, the exact same
thing is going to happen. All of our digits are going to
shift one place to the left. So, right now, we have a
three in the ones place, we have a zero in the tenths place, we have a one in the hundredths place, and we have a five in the thousandths, thousandths place. But now, they're all going to
shift one place to the left. So, the three is now going
to into the tens place, it's going to shift one place to the left. So, we're going to
have, in the tens place, we are now going to have our three. And now, this zero, which
was in the tenths place, is now going to shift into the ones place. So, the zero is going to go right over, the zero's going to go right over there. That is now in the ones place. And them we'll put our decimal. And now, what's going to
go into the tenths place and the hundredths place? And actually, I'll rewrite
the thousandths place as well. This one is going to shift
one place to the left, into the tenths place, so into the tenths place. And then the five that was
in the thousandths place is now going to shift
one place to the left, into the hundredths place, so into the hundredths
place, just like that. Now, we could put a
trailing zero over here, but that's not going to change
the value of this number, so I'll just leave it like that. And so, there you have it. We see that every digit has
shifted one place to the left. So, this is equal to 30.15. I'll just put that zero there for kicks. And so, we could think
about the other way around. What if I were to take 67.5, and if I were to divide it by 10, or another way of thinking about it is if I were to multiply this by 1/10. Pause the video, and see if you can figure out what that's going to be. Well, now, every digit is going to shift one place to the right. So, the six is going to
be in the ones place, the seven is going to go
into the tenths place, and then the five is going to
go into the hundredths place. So, let's write that out. So, I have six is going
to go into the ones place, and then we're gonna
have our decimal point, our seven is going to go
into the tenths place, and then our five is going to
go into the hundredths place. So, there you have it, we get 6.75.