Learn how to add numbers to make the number five. Created by Sal Khan.
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- At0:15can't you just do 5 - 3 to find the unknown number?(4 votes)
- Could this be a good start for "solving for x" learning?(1 vote)
- This is a very basic form of equations in one variable, so i think it is the starting point for 'solving for x' or solving equations for variables type of learning.(7 votes)
- How do you know thats addition ? Right now it looks like Subtraction to me.(1 vote)
- Addition is when you are adding one number to another through this sign: +. Subtraction is when you are taking away a particular amount from a bigger number through this sign: -.(7 votes)
- Couldn't you just add the 3 and the 2 together instead of subtracting?(2 votes)
- You can, but you need to have a pretty good grasp on adding and subtracting with integers of a different sign ( +2, -3 ) so that when doing problems like this you don't create bad habits and repeat the same mistake over and over again with no one to tell you that it is right or not.(4 votes)
Voiceover: What I want to do in this video is think about the different ways to make five. What do I mean about that? Right over here we have three plus something, so there's some unknown number right over here. Three plus something is going to be equal to five. In this diagram we have three circles here that represents the three and then we have five boxes. One, two, three, four, five. How many more circles do we need in order to fill up all the five boxes? We've clearly already filled three. Well, let's think about it. We just have to add one and two circles. Now we have a total of five. Three blue ones and two of this purple color or I guess this pink color. We see that three plus two is equal to five. That mystery number here, that blank is two. Three plus two is equal to five. Now over here we have two plus a mystery number is equal to five again. Once again, we have five spaces and two of them are filled in with this kind of purple, pink stars. How many more do we have to fill in in order to fill in all five boxes, two plus what is equal to five. I encourage you to pause the video and think about it. I'm assuming you've had a go at it. Let's think about how many more of this boxes we need to fill in, in order to fill in all five of them. We need to fill in one, two, and three of the boxes in order to fill out all five. Now we have five stars. We have two of the pink ones and we have three of the blue ones. We can say that two plus three, the mystery number here is three. Two plus three is equal to five. Let's do a few more of this. Here I have four plus some mystery number is equal to five. I filled out four of the five boxes, how many more do I need to fill in in order to have five things, in order to have all five boxes filled out. Well we see very clearly here that there is exactly one box that we need to fill in or one box that we need to add to the other four. We see that four plus one is equal to five. Now let's go the other way. We have the five boxes, only one of them is filled in. How many more of these smiley faces do we need to add, in order to fill in all the boxes? Or another way of thinking about it, we have one, how many more do we need to add to one to get to five? Like always, I encourage you to pause the video and think about it on your own. Well, let's see, we can add one, we can add two, we can add three, and four yellow smiley faces. We could say one, one we have one white smiley face plus one, two, three, four yellow smiley faces is equal to five. You see a pattern here. Let's start at the beginning. Three plus two is equal to five, and two plus three is equal to five. When you had three filled in, you needed two more to get to five. When you had two filled in, you needed three more to get to five. Notice three plus two, two plus three. Same thing over here, four plus one is equal to five, one plus four is equal to five.