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### Course: MAP Recommended Practice > Unit 14

Lesson 25: Multiply by tens word problems# Multiplying by multiples of 10

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?(255 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.(193 votes)

- so we should just add a 0 when muitpliling by 10(5 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.(3 votes)

- For 60x7, can we just multiply 6x7 and then add a zero at the end?(4 votes)
- that's a good idea for numbers with only one zero. When a number has TWO zeroes (for example: 200 x 2), you should mulitply 2x2 and add TWO zeroes. Just so you won't get confused.(4 votes)

- this is hurting my brain(5 votes)
- so 50 x 7 is the same as 50 + 50 + 50 + 50 + 50 + 50 + 50?(4 votes)
- yes, the answer to 50 x 7 and 50 + 50 + 50 + 50 + 50 + 50 + 50 both equal 350.(3 votes)

- do we haft to watch this?(3 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)

- but how this isn't the correct process like this the answer would 100 percent be different(4 votes)
- why cant we just multiply 5x7 and add a 0 to the end(1 vote)
- You can! And that's exactly what Sal is saying in the video. For example, for the second question, at1:16:

1. Start with the original question, 50×7.`50 × 7`

2. Remove the zero from 50 by splitting it up into 10×5.`50 × 7 = 10 × 5 × 7`

3. Now you can multiply (5×7) to get 35, as you suggested.`10 × (5 × 7) = 10 × 35`

4. Finally, you add the zero to the end, which is the same as multiplying by 10.`10 × 35 = 350`

(2 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.