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## 3rd grade

# Multiplying by multiples of 10

CCSS.Math:

Sal decomposes multiples of ten to multiply. Created by Sal Khan.

## Want to join the conversation?

- why cant we just multiply 5x7 then add a 0 to the end?(247 votes)
- for 50x7? You can. He's just explaining the underlying process here. With enough practice, you will be able to easily shift 0's around given a certain power of 10.(184 votes)

- 1:49why didn't you just multiply the thing then just working it out the long long long way!?(2 votes)
- Well, he is just doing this because it is gonna give you a better comprehension if he does a harder problem. I mean, if he did 1*7 (1x7 or 1 times 7), you would not learn anything because you already know the answer, and it won´t help you at all.(5 votes)

- why cant we just multiply 5x7 then add a 0 to the end?(3 votes)
- you can but he does it so it will be clear to the watchers (what I mean is that it is people watching)(1 vote)

- so we should just add a 0 when muitpliling by 10(4 votes)
- Yes your right also when multiplying by 100's 1000's etc. You just add the 0's to the number that you are multiplying by.(2 votes)

- why 4x9 really hard? i dont undrstand this thing. if someon can help me pls help(0 votes)
- 4 x 9 = 36. Multiplication is repeated addition. If you have problems multiplying a number, try adding that number to itself. For example, for 4 x 9, you could either add 4 + 4 = 8 + 4 = 12 + 4 = 16 + 4 = 20 + 4 = 24 + 4 = 28 + 4 = 32 + 4 = 36 or 9 + 9 = 18 + 9 = 27 + 9 = 36. Another strategy would have been multiplying and then subtracting: 4 x 10 = 40 - 4 = 36.(13 votes)

- keyanna is right why can`t we do that(4 votes)
- not always for example 50&50(3 votes)
- i put this all on my note book thank khan(3 votes)
- Can we say that 50x7 is (1/2 (=one half) times 100) x 7 what is the same thing as half times 700? We'd still get 350, what is the right answer here.(3 votes)
- Yes, both ways work.

If you want to use**100×7**to calculate 50×7, all you need to do is add a "times one-half" somewhere in the expression (since 50 is half of 100).

So,**100×7**.*× 1/2*= 700*× 1/2*which is equal to 350

If you were to just calculate 50×7 as-is, you'd also get 350 (35 with a zero added on).

I hope this answered your question :)(1 vote)

- All you had to do was multiply 50 x 7 right? You see the number 5 and 7 and you'll think that oh! I think we need to multiply 5 x 7 and get the answer of 35! And then i will add my zero in and get my answer of "350".

See? Ez math.(3 votes)

## Video transcript

Let's see if we can
figure out 3 times 60. Well, there's a couple of
ways you could think about it. You could literally view
this as 60 three times. So you could view this
as 60 plus 60 plus 60. And you might be able to
compute this in your head. 60 plus 60 is 120,
plus another 60 is 180. And you'd be done. But another way to think
about this is that 3 times 60 is the same thing as 3
times-- instead of thinking of it as 60, you could
think of 60 as 6 times 10. 3 times 6 times 10. Now, when you're multiplying
three numbers like this, it doesn't matter what
order you multiply them in. So we could multiply
the 3 times 6 first and get 18 and then
multiply that times 10. And 18 times 10 is
just going to be 180. It's going to be 18
with another zero. So this is going to be 180. Now, the more practice you
get here, you'll realize, hey, I could have just
said 3 times 6 is 18, but I have to worry about
this 0 right over here. So I'm going to put one
more zero at the end. It's going to be 180. Same answer that we
got right over there. Let's do another one of these. So let's say we want
to multiply 50 times 7. And I encourage you to pause
the video and think about it yourself, and then unpause
it and see what I do. So 50, well, there's a couple of
ways you could think about it. One, you could literally
try to add 50 seven times. Adding 7 fifty times
would take forever, but you could literally say 50
plus 50 plus 50 plus 50-- let's see, that's four--
plus 50 plus 50. Let's see, that is six. I'll do one more
right over here. 50 right over here. So this is 50 seven times. If you add together 50 plus
50 is 100, 150, 200, 250, 300, 350. So you could do it that way. But you could imagine that
there is an easier way to do it. You just need to
realize that 50 is the same thing as 10 times 5. So we could write
this as 10 times 5, and then we're
multiplying that times 7. Once again, the order that
we multiply does not matter. So we can multiply the
5 times the 7 first. We know that that
is 35, and we're going to multiply that times 10. 10 times 35, well,
we're just going to stick a zero at
the end of the 35. It's going to be equal to 350. Now I want to do that
zero in that same color. It's going to be 350. Now, you might
realize, hey, look, I could have just looked
at this 5 right over here, multiplied the 5 times the
7, and have gotten the 35. And then, not forgetting
that it's actually not a 5, it's a 50. So I have to
multiply by 10 again, or I have to throw
that 0 at the end of it to get that 0 right over here. So 50 times 7 is 350.