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More about fractions

Sal shows fractions as parts of a whole and parts of a group. Created by Sal Khan.

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Video transcript

- [Sal] What I want to do in this video is think about the idea of a fraction. And for the sake of this video as we just learn what a fraction is, you can really think of it as a part of a whole. Later on we'll think of it in even more ways. What do I mean by a part of a whole? So let's imagine that we have a pizza. I will do it in a pizza colored. . . so I'll do it an orange I guess that's roughly the color of a pizza. Pizza I guess is yellow and kind of red for the tomato sauce so yellow for the cheese. So let's say this was a perfect circle. I'm gonna try my best to draw a circle right over here. And let's say that I divided into four equal parts so just like that I've divided it into two equal parts and now I will divide it into four equal parts. So there's a total of four slices, four equal slices of this pizza. Four equal slices of this pizza. And let's say on three of the slices I have cheese only cheese. only only cheese. And lets say on this fourth slice, I have cheese and, I don't know, I have olives I have olives right over here. So we can ask ourselves, "What fraction of this pizza has olives on it?" Well, we have four total slices. They all are equal slices. We have four equal sections here and one of those equal sections has olives, so we could say one-fourth of this pizza one-fourth of the pizza the pizza, has, has olives. One out of the four. Now someone said, "Well what fraction of this pizza only has cheese?". Well this fraction has cheese, but it also has olives so we won't count that. Then we have one, two, three, three fractions three of these equal pieces have only cheese so we could say three out of a total of four pieces of. . . so three fourths of the pizza has only cheese. Let's do a couple more examples because this is probably one of the most important and most useful ideas that you can have in your brains. Let's do another, another few examples. So this is what we sliced up, a pizza. Let's imagine instead that we're looking at a, some animals of some. . . I'd say fruit. Fruit is easier for me to draw So let's say that I have, I have an orange so that's an orange right over there, and I have a Banana. So that's my best attempt at drawing a banana right over here. And I have a lemon. I have a lemon. Just like that. And let's say that I have an apple. I'll draw. . . Actually I'll draw an Apple. It's a little bit of a strange color for an apple, but we'll go with it. So that is my apple right over there, and actually let me draw one more, let me draw a grape. Or grapes. So I have a bunch of grapes right over here. And if I were to ask you, at least the way that it is drawn, "What fraction of my fruit that I have is. . ." Let's say, ". . .is yellow?" "What fraction of my fruit as drawn is yellow?" Well, we could say, we have a total of one, two, three, four, five pieces of fruit So there's a total of five pieces of fruit. And then what fraction of those pieces of fruit are yellow? Well, the way I drew it, I have two yellow pieces right over here. So we'd say, "Two fifths of the fruit is. . ." and it's strange that I'm writing this in white so I'll write it in yellow ". . .is yellow." Two fifths is yellow. Let me do one more example to just really make the point clear. So let me draw a candy bar, and this is all food-related. I guess it's getting close to dinnertime. I apologize. So let's say I have a candy bar. Just like that. And let me divide it into let me divide it into 5 equal pieces 1 one, two, three, four that doesn't look too equal. So the whole point is to do it equal, one, two, three, four that looks pretty close. So let's assume that these are equal pieces. So my drawing isn't perfect but let's assume all of these pieces are equal and I'm hungry so let's say that I eat I eat this piece, and I eat this piece so they essentially go away. So what fraction of the pieces have I eaten? Well, I have a total of five pieces. There's a total of five equal sections of this candy bar I guess I could say. And I ate two of them, so two fifths two-fifths of this bar, so this is, I've taken them away two-fifths of the bar have been eaten. Have been eaten. So I'll leave you there, maybe I'll make another video that doesn't have as many food analogies, but this is a super useful concept and I encourage you to think about this as much as possible and then try the exercises on Khan Academy