Early math review
Adding four two digit numbers
Sal uses place value to add four two digit numbers. Created by Sal Khan.
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- [Instructor] What I wanna do in this video is try to figure out what 35 plus 22 plus 10 plus 16 is equal to. So pause this video and see if you can figure that out. All right, now let's work through this together. Now, as you will learn, there's many ways to approach this, but the way I'll think about it is by thinking about place value. For example, when we look at 35, we have 3 tens and 5 ones. 3 tens, right over here, that's the same thing as 30. So you could view 35 as 30 plus 5. And we can do the same thing for 22. 22 is 2 tens, which is 20, that's what that 2 tells us, plus 2 ones. So 20 plus 2. And then we have 10, which is just 1 ten and no ones. So plus 10. And then 16, we could rewrite that as being the same thing as 1 ten. So 10 plus 6 ones, plus 6 ones. Now, why was this useful? Now I could take all of the things that are a certain number of tens, so 30, 20, 10, and 10, and try to add those together. And then separately, I will try to add the 5, the 2, and the 6. And when you see when I do it, I think it'll make a little more sense why it's useful. So, we have a 30 plus 20 plus 10 plus 10, plus that purple 10, plus that purple 10. So I've already accounted for the 30, the 20, that 10 and that 10. And then I'll have all of the things that were written in the ones place before. So plus 5 ones, so I got that taken care of, plus 2 ones, that's taken care of, plus 6 ones, plus 6 ones. And now all of these we can think about. What is 30 plus 20? Well, 30 plus 20 is going to be 50, and then 50 plus 10 is 60, and then 60 plus 10 is 70. So that's all going to be 70. Another way to think about it is we have 3 tens plus 2 tens plus 1 ten plus 1 ten. 3 plus 2 plus 1 plus 1 is 7, so we have 7 tens. And you actually saw that up here as well, 3 tens plus 2 tens plus 1 ten plus 1 ten, that's going to give us 7 tens. And then we can add this other part where we see 5 plus 2 is 7 plus 6 is 13. So it's 70 plus 13. Now, what is 70 plus 13? Well, I could rewrite 13. I could do something similar as 10 plus 3, And that makes it a little bit more obvious. We can see that that's the same thing as 70 plus 10, which is 80 plus 3, which is the same thing as 8 tens and 3 ones, which is equal to 83. And we're done.