Sal solves addition and subtraction word problems.
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- does this count as clearing a variable? something like pre algebra ?(26 votes)
- I have always had trouble with math and understanding but one question How would set up a problem for a subtracting. I have seen people set it up two ways for an example the one you did for 20- 5 ive seen it both ways and subtracted two ways with burrowing and not borrowing which way is the correct way?(14 votes)
- 20 - 5 does require borrowing. Whenever a number goes under 0, it always has to borrow from it's big brother. 0 - 5 goes under 0, so it has to borrow from it's big brother, the two. 2 - 1 = 1, and now the 0 becomes 10. 10 - 5 = 5 and 10 + 5 = 15! The answer is 15! I hope this helps!(12 votes)
- There were some gorillas lifting weights in the gym. Then, three more gorillas came and started running. Now, 17 gorillas are in the gym. How many gorillas were lifting weights? So, that's the number of gorillas that were originally in the gym, the gorillas lifting weights. So, how many gorillas were lifting weights? They have this diagram for us. Let's see, what is this diagram telling us? How many squares are there? There's one, two, three, four, five, six, seven, eight, nine, ten, 11, 12, 13, 14, 15, 16, 17 squares. So, this 17, this is how many gorillas there are now in the gym. So, this is the 17 right over here. How many gorillas, how many gorillas there are now in the gym. How many gorillas there are now in the gym, that's the 17. Now, there are some gorillas in the gym. Three more came and started running. So, maybe these are the three that came in and started running. So, one, two, three. So, how many were there to begin with? How many there were there lifting weights? So, all the rest of these must have been there lifting weights. So, that is one, two, three, four, five, six, seven, eight, nine, 10, 11, 12, 13, 14. So, 14 gorillas were lifting weights. That's the ones where I wrote the numbers in this, in this purplish color. Then, three more came in and started running and now, there are 17. You see the 17 blocks. So, how many gorillas were lifting weights to begin with? Well, that's the 14. That is, that is the 14. Now, there's other ways that you could have done this. You could have said, okay, there were some number of gorillas that were lifting weights in the gym. Now, three more gorillas walk in. Three more gorillas walk in and now they're going to be, they are now going to be 17 gorillas. So, you could say okay, something + 3 is 17 or you could say 17 - 3, minus the three that walked in, well that's going to give you how many you originally had. How many you originally had and either way you think about it, you're gonna get to, you are going to get to 14. 17 - 3, you can view that as well, this 17 is one 10 and seven ones. So, you take away three ones. 7 - 3 is four. So, 17 - 3 is gonna be 14. Let's do another one of these. This is more of what you might see on Khan Academy and it's good to just know what they're asking for. So, this says, I bought a bag of 13 mini chocolate chip cookies. My sister ate some of them and I was left with 8 cookies. And I was left with 8 cookies. How many cookies did my sister eat? So, I had a bag of 13 cookies. I had a bag of 13 cookies. My sister ate some of them. So, those cookies are going away. So, I'm gonna subtract those cookies. So, my sister ate some of them. So, that's the question mark. How many cookies did my sister eat? She ate some of them and then I was left with 8 cookies. Then, I was left with 8 cookies. So, 13 - something is = 8. Or you could view this as, you wanna solve for the something, you could say that 13 and you, some of you might just say okay, I know 13 - 5 = 8, so you would know that the question mark is equal to five. Or you could say that 13, 13 - 8, minus 8 is equal to ?. Is equal to ? Or, you could even say, well if I was left with 8 cookies, you could say if I was left with 8 cookies and my sister ate some of them. So, this is what I'm left with plus the ones that my sister ate, that must be how many cookies I originally had. That must be the number of cookies I originally had. So, any way you do it, you get your question mark is equal to five. Sister ate 5 cookies. Now they ask us which choices can help you solve the problem? Well, all of these, this is, now you know the question mark is five. We know that 13 - 5 is equal to 8. So, let me write that down. We know that, actually let me just replace this. Let me just replace this with a 5. So, 13 - 5 = 8. Well, that's a true statement that's describing what just happened here. So, this could help me. Actually, not that one. That's 13 + 5. 13 - 5 = 8, that's this right over here. And that is a useful thing to know because look, if I started with 13 cookies, my sister ate 5 of em, I'd be left with 8. Now, what about this one over here. 13 - 8, well we know this would be equal to 5 now. They don't have a 13 - 8 = 5, so we don't have to select that. Then here, we now know the question mark is 5. So, 8 + 5 is 13. 8 + 5 = 13, that's useful too. 'Cause we know 8 + 5 is 13, then we know what I'm left with plus what my sister ate is 5, is going to be the 13 that I started with. 13 + 5 = 18, not so useful. 18 has nothing to do with the problem. There weren't, ya know, somehow, it's not like my sister somehow baked five more cookies and added them and now I'm left with 18 cookies. I'm left with eight cookies now and so I didn't add the 5. She ate those cookies. So, I would subtract them.