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Teens as sums with 10

Sal looks at the 1 in each teen number and thinks about what it really means.  Created by Sal Khan.

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Video transcript

Let's count together from zero to 19. So, we have zero, one, two, three, four, five, six, seven, eight, nine. Now, we go to the two-digit numbers. Then, we go to 10, 11, 12, 13, 14, 15, 16, 17, 18 and 19. Now, why did I do this? The whole point of this is to see a pattern here, especially in these two-digit numbers. What do we see is true about all of these numbers? You could think of them they're being in their teens. They all, you might see, have a one as their first digit, and that one is sitting in what we'll call the tens place. and literaly says that each of these numbers that one represents a 10. What do I mean by that? If we go up to 10 over here, this is telling us that this one represents 10, and this zero represents zero ones, so 10 is 10 plus zero. You say, "Oh, well, what's the big deal? "We already knew that any number plus zero "is that number again." I think you'll see the pattern as we go into these other numbers. So, 11 is equal to - so the yellow one represents 10, and this pink one represents one, and then 12 - 12 - this yellow one represents 10, and this pink two represents two. You could think of it as this yellow one says this is one 10, so we have a 10 right over there. This is two ones, and you have two right over there. We can keep going. I think you see the pattern. 13 is 10 plus three. 13 is 10 plus three. 14 is equal to 10 plus four, plus four. 15 is equal to 10 plus five. I'd encourage you to pause the video now, and try to finish the rest. Try to write 16, 17, 18 and 19 as 10 plus some other number, and really just make sure that you understand the pattern that's happening. Let's do that. So, 16 - so I can even write them all like this. They're all going to be 10 plus something, so 16 is 10 plus six. 17 is 10 plus seven. 18 is 10 plus eight, and 19 is 10 plus nine. When you write it out like this, it becomes pretty clear that this one in each of these numbers represents 10, and the digit on the right - that just represents that many ones.