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## How 10 relates to place value

Current time:0:00Total duration:7:01

# Multiplying whole numbers by 10

CCSS Math: 4.NBT.A.1

## Video transcript

- [Voiceover] Multiplying by 10 creates a really neat pattern with numbers, so let's try a few out and see if we can discover the pattern. Let's try to figure it out. We'll start with one that
maybe we already know, let's start something like two times 10. And maybe we know the
solution but let's think about more than just the solution, let's think about what it really means to multiply two times 10. Two times 10 means we have two 10s. Or, we have a 10, plus another 10. Which is equal 10 plus
another 10, is equal to 20. And again, maybe we didn't
need this middle part, maybe we already knew two times 10 is 20, but it'll be helpful to think about what two times 10 means because it will help us when we get to much trickier ones. Let's try one that's maybe
just a little bit trickier, let's try five times 10. Five times 10 is going to be five 10s, or, one 10 plus another 10 plus a third 10 plus a fourth 10 plus a fifth
10, so we have five 10s, we have one, two, three, four, five 10s. And we can solve that, we
can add those together, 10, 20, 30, 40, 50. So our solution here, is 50. Five 10s is 50, or five times 10 is 50. Let's go to one more, maybe
one that we don't know the answer to off the top of our head, let's try something like 13 times 10. Maybe we don't know the
answer to 13 times 10 but we do know that 13 times 10 is 13 10s. And we can count 13 tens. We have a 10, plus another 10,
plus another, there's three, four, five, six 10s,
seven, eight, nine, 10, we're almost there, 11 10s, 12 10s, and finally a thirteenth 10. Let's count to make sure. One, two, three, four, five,
six, seven, eight, nine, 10, 11, 12, 13, great. So we have 13 10s, so let's count those, let's see how much that is. We have 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130. So our solution is 130. I think we can pause
here and look at the ones we've done so far and see if
we can figure out this pattern. Two times 10 is 20, five times 10, 50, 13 times 10, 130. In every case we took the
number that we started with, and we kept that, that
was part of our answer, and then we added one new thing, which was a zero at the end. Any time we multiply a whole number by 10 we're gonna keep the original number and simply add a zero to the end. So let's try some without
all this middle stuff. What if we had something
like, a little bit tougher, maybe like 49 times 10. Well 49 times 10, you may guess is, is gonna be a 49 with
what at the end, a zero. Or 490, because this would be 49 10s, and if we counted 49
10s we would get to 490. Let's go even tougher than that. What if we had something
like 723 times 10? Well our solution, and
maybe you've guessed it, is going to be 723 with a zero on the end, or, 7000 200 and 30 is the way we would actually read that answer. But looking at it, it quite
simply is the original number, 723, with a zero on the end. Let's take this one step
further and let's think about it in terms of place value. Let's think about what
multiplying by 10 did to these numbers in terms
of their place values. So here we have a place value chart, and let's start back with one
of the simpler ones we did, like two. Two is two ones. And when we multiply that two times 10, the two moved up a place value. And then we had to fill
in this empty place value with a zero. So two times 10 was 20,
and what happened in terms of place value, was the two
moved up one place value, it went from ones to tens, it moved to the left one place value. Let's look at another one,
maybe one of the tougher ones, something like 723. When we multiply 723 times 10, we multiplied this one times 10, the seven moved one
place value to the left to the thousands, the two
moved up to the hundreds, and the three moved up to the tens. And then again, we simply added a zero in the empty place value in the end. So multiplying by 10, one
way to describe the pattern, is that it adds a zero to
the end of a whole number. But another way to describe the pattern, is that it moves every
place value, every number, one place value to the left. So if we tried one looking at it this way, let's say we tried something
like, 20, let's do 27. If we multiply 27 times 10, well the two's gonna move
one place value over, so it will now be in the hundreds, and the seven's gonna
move over a place value, so it will be in the tens, and then we'll have to fill in that empty place value with a zero. So we can say that 27
times 10 is equal to 270. So whether we think about
it as adding the zero to the end or moving the
place value to the left, multiplying by 10 creates
a really neat pattern, that we can use to help us
to solve these problems.