Adding up to four 2-digit numbers
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- [Instructor] We're told that Tomer has a bookshelf. The table below shows how many books are on each shelf. So the first shelf has 19 books, the second shelf has 24 books, the third shelf has 32 books, and the fourth shelf has 7 books. How many books are on the bookshelves all together? So pause this video and see if you can work that out before we work through it together. All right, now let's work through it together. So if we wanna figure out how many books are on the bookshelves all together or on the bookshelf altogether, we essentially have to add 19 books to 24 books to 32 books to 7 books. So really, this is going to be 19 plus 24 plus 32 plus 7. That's what we really have to figure out. Now, how can we do that? Well, I like to do that by separating out the tens place from the ones place. For example, I could rewrite 19 as 10 plus 9. I have one 10 and 9 ones, one 10, 9 ones. I could rewrite 24 as 20 plus 4. So plus 20 plus 4, I could rewrite 32 as 30 plus 2. And last but not least, I could rewrite, let me find a nice color here, maybe I'll use blue, I could rewrite 7. Well, actually, I want won't rewrite 7. 7 only has 7 ones, so I'll just write 7 right there. And now what I can do is I can take each of these that came from our tens places right over here. So I would have 10 plus 20, I'm just crossing it out so I can keep track of things, plus 30, plus 30. So I did these three, and then I would have all of the ones or the things that I got from the ones place, and let me do that in orange. So it's the 9, the 4, the 2, and the 7. So plus 9 plus 4 plus 2 plus 7. Now, what's 10 plus 20 plus 30? 10 plus 20 is 30, and then that plus 30 is going to be 60. Another way to think about it, I have one 10 plus another two tens, which would give me three tens, plus another three tens, which would give me six tens, so that's 60. And then how do I think about this? Well, I can do this in any order. I can see that 2 plus 7, that's going to be equal to 9, and I can see that 9 plus 9 is equal to 18. So this is going to be the same thing as 18 plus 4. And then let's see it takes two to get to 20, and then I have another two. Actually, let me write it that way. So I could also write this as 60 plus, and if I just take 2 from the 4 and put it onto the 18, this would be 20 plus 2, this and this are equivalent. And the reason why I did that is it's easy now to add 20 to 60. 20 to 60, that's two tens plus six tens, that's going to give us eight tens, that's going to be 80. And then 80 plus 2 is, of course, going to give us, and we can have something of a drum roll, almost have the answer, 82. That's how many books Tomer has all together.