Strategies for adding and subtracting within 100
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- What I want to calculate in this video is 53 plus 17, plus 17. And the way that I want to do it is by breaking up the 53 and then rearranging the numbers, and you'll see what I'm talking about in a second. So 53 plus 17, that's going to be the same thing as, I'm gonna break up 53, essentially into the tens and the ones. So 53 can be rewritten as 50 plus 3. That's 53 right? And I could put parentheses around it like this. So these curvy things, so this is an open parenthesis and this is a close parenthesis. This just means to do whats inside the parentheses first. So 53 is the same thing as 50 plus 3 and we still have the plus, we still have the plus 17. Now, I could just add the 50 plus 3 first and then add the 17 and that would just get back there, but we want to figure out what this whole thing is. So I could rewrite this, this is the same thing as 50 plus 3 plus 17, plus 17, and I could do the 3 plus 17 first. So here I'm doing the 50 plus 3 first, I have those in the parentheses. Here I'm doing the 3 plus 17 first, and this is just a property of addition that I could do this. I could change the order in which I'm adding. So here I'm adding 50 plus 3 first. Here I'm going to add 3 plus 17 first. And so what's this going to be equal to? Well this is going to be equal to I have my 50 here, I have my 50 plus, now whats 3 plus 17? What's 3 plus 17? Well 3 plus 7 would be 10, so 3 plus 17 is going to be 20. So it's going to be 50 plus 20. Now what's that going to be? Well it's 5 tens plus 2 tens, which is going to be equal to this is just going to be equal to 7 tens, or 70. Let's do another example, and this is going to be one like the type of exercise that you might see on Khan Academy. So let's, let me take, open that up. So they'll say "Complete to solve 41 plus 9" so once again we can look at, we can look at the 41 and we can break it up. 41 is the same thing as 40 plus what? Well, it's 40 plus 1, and of course then you have the plus 9 there and I can do the 9 see this plus 9 is this plus 9 there. Now they change the order in which we're adding. So 40 plus 1, that's in the parentheses and then you add 9, well that's the same thing as 40 plus well, that's the same thing as 40 plus 1 plus 9. So here we're doing the 1 plus 9 first, and if we do the 1 plus 9 first what do we get? Well, 1 plus 9, 1 plus 9 is what? Well, that's just going to be equal to 10. Notice what this, this box is replacing what was in the parentheses before, so in the parentheses you had 1 plus 9, well that's 10. And what's 40 plus 10? Well it's 4 tens plus 1 ten so that's 5 tens. Or, 50, and we're done.