Sal adds 7 + 6 using the number line. Created by Sal Khan.
Voiceover:Let's think about what seven plus six is, and I encourage you to pause this video and think about it on your own. I'm assuming you've given a go at it. Let's think about it. We could do this as seven objects plus six more objects, and then think about how many total objects we had. For example, we could view it as seven, say, tomatoes, so one tomato, two tomatoes, three tomatoes, four tomatoes, five, six, and seven, and then to that, we're going to add six more objects. Let's say they're blueberries, and we care about the total amount of fruit. So, one, two, three, four, five, and six - six blueberries. Now, how many total pieces of fruit do I have in all now? I started with seven, so that's seven. If I keep counting, this is seven so this is going to be eight, nine, 10, 11, 12 and 13. I now have a total of 13 pieces of fruit - 13 pieces of fruit. What are other ways that we could have thought about this? We could have also done it on a number line. Let's draw ourselves a number line. Let me do this in a color that I haven't used yet, actually. Let's say I have a number line just like that. I can start the number line at seven. I could start at seven, and I'm going to add six more. I'm going to move six more up the number line. This is one, two, three, four, five, six, and, of course, I could keep going. It's going to be eight, nine, 10, 11, 12, 13. Of course, you could keep on going. Seven plus six - you could visualize this as starting at seven, and then making six jumps up the number line - one, two, three, four, five, six. Either way, we get to 13. Another way to think about it is, look, we started with seven. We added three to get to 10, and so we have to add another three, which gets us to 13. That really goes to the heart of what the number 13 represents. The number 13 has a one as its left digit, This digit is in the tens place, so it literally represents one 10. So, it's one 10 plus three, plus three ones. You see that right over here. When I added the pieces of fruit, this right over here is one group of 10. So, that's one group of 10. We had to add three to get to that one group of 10. We kind of filled that bucket, and then we had three more, so when you add seven plus six, you fill one whole group of 10 and then you have three ones left over. This is the three ones right over here. Another way you could think about it is seven plus six is the same thing as 10 plus three, which, of course, is 13. This is the same thing - one 10 is 10, plus three, plus three ones - 13 either way you look at it.