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## 2nd grade

### Unit 1: Lesson 4

Add and subtract within 20- Adding within 20 using place value blocks
- Adding within 20 using ten frames
- Subtract within 20 using place value blocks
- Subtract within 20 using a number line
- Add & subtract within 20
- Addition and subtraction word problems: gorillas
- Addition and subtraction word problems: superheroes
- Add and subtract within 20 word problems

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# Addition and subtraction word problems: gorillas

Sal solves addition and subtraction word problems.

## Want to join the conversation?

- does this count as clearing a variable? something like pre algebra ?(25 votes)
- Yeah, this is like solving really simple equation.(0 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?(12 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!(9 votes)

- Question,

Why is the variable so important in math(9 votes)- Variables are very (even extremely) important in math because they are placeholders for unknown values in an equation, function, etc. For example, if we have the equation 2x=6, we can solve for the variable (and find that x=3). But without variables there is no way to do that. We would have to directly solve for 6÷2. Though interpreting the calculation that way is not difficult, imagine if you have several different values performing different operations, requiring us to rewrite and rearrange the order of several things.

In functions, variables provide the same thing and convenience. For a function, such as y=2x, we can use it to help see what y will equal for particular x values (note that both y and x are variables). But without variables, we would have to solve each equation one by one:

• 2x1=2

• 2x2=4

• 2x3=6

• 2x4=8

• 2x5=10

• And so on in a pattern of 2 times the next value in a never-ending list

So variables provide a very big convenience in writing calculations, functions, tables, and other sorts of things.

[R](4 votes)

- Why would the 1st task be written in a way that makes the end result inconclusive? As it is stated there we do not know whether all or only ''some'' of the 14 gorillas were actually lifting weights.(4 votes)
- When you get a question that is ambiguous you have to see how the problem should be worded to make a question that allows us to write equations. In this case the problem says "some"; now, can you use that to solve a problem?, I don't think so, it is ambiguous. If you use "all" instead, you can start solving it.(3 votes)

- no, I don't understand, my question was; Raj and Carlos are reading. Carlos read a chapter in 4 minutes, which is 3 minutes less time than it takes Raj.(3 votes)
- To answer your question you have three pieces of information:

- you can find who took less time and who took more time to read. Draw it on a piece of paper if it helps.

- You also know how long one of them took to read,

- You know the difference in reading time between the two.

With all that you should be able to find out how long the other one took to read.(5 votes)

- i'm not good at math i'm slow(5 votes)
- do not say that you should just trust in yourself OK(1 vote)

- how do you now if it is addition or subtration(2 votes)
- adition is + and subtraction is -(1 vote)

- How many gorillas were in the gym first(3 votes)
- This is hard for me i am a bart
`crys`

(2 votes) - So listen, I understand what you are saying. I think what most people would get confused by this reason. When the question asks, "Which choices can help you solve the problem?", select all that apply. I think generally people would stick with the middle one because the last one doesn't actually help solve the problem, it helps check to see if the problem is right. That's what I'm confused about and frustrated and hope that you guys just went with the middle one, almost like making us feel dumb when your really smart. I hope you understand because I don't see how else that would help solve this question, "Which choices help you solve the problem?", Answer of course 13-5=8 than later on to check so on so on the bottom one. Thanks....(2 votes)

## Video transcript

- 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.