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## Early math review

### Course: Early math review > Unit 3

Lesson 1: Teens# 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.

## Want to join the conversation?

- Why does it say 11 and 12 are teen numbers? They don't have teen in it's name.(7 votes)
- Teen numbers are numbers in the range of 11-19.(17 votes)

- why something over zero is infinity?(8 votes)
- It's undefined. You can try to divide something, say number 5 by 0,0001 you would get a very high number. You may then be tempted to think that the closer the denominator gets to zero the higher the result is going to be therefor going to be infinity.

However try the exact sam number 5 and divide it by -0,0001 you would get a very large Negative number. Therefor you could equally conclude when the denominator closes in to zero from the negative side you would reach negative infinity.

In short: The argument that dividing by zero is infinte is wrong, it IS undefined.(14 votes)

- Will they ever be a bigger place value chart? So if I have 789456123 what would be the place value chart(5 votes)
- The point of a place chart is to help you count numbers. Eventually you won't need place charts anymore. But if there was a place value chart and the number 789456123 was in there, then the place value chart would begin at 100, 000, 000 and end at 999, 999, 999.(10 votes)

- what is the history of numbers(8 votes)
- From my understanding, there is no true origin of numbers that historians can claim to be. Trying trace the humanity history, number existed way before as a means of communication and measurement just that it was interpreted differently in different societies. Take a look from the historian point of view. Ancient greek symbols like pie, alpha, beta e.t.c and the roman symbols came to interpreted as numbers in the modern contemporary English. Good question.(5 votes)

- why did you only count to 20?(5 votes)
- because i don't know the rest of the numbers(2 votes)

- I dont understand comen core math why do they make it so hard to awnser the simplist problem(6 votes)
- Why do you have to watch videos(4 votes)
- Watching videos helps you better understand what's happening.(7 votes)

- some math is inpossible but you can try to do the math so your grades can be up(4 votes)
- So you can lean later in the future so you know.(3 votes)

- is there a stop in a very large number?(4 votes)
- Nope but you can write it the way you want.(1 vote)

- Why do we use zero not roman numerals(3 votes)
- Because they take longer to write e.g. 1987 can be 1987, but in roman numeral it is MCMLXXXVII(3 votes)

## 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.