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## Arithmetic (all content)

### Unit 2: Lesson 9

Strategies for adding and subtracting within 100- Adding 53+17 by making a group of 10
- Add 2-digit numbers by making tens
- Adding by making a group of 10
- Add 2-digit numbers by making tens 2
- Strategies for adding 2-digit numbers
- Select strategies for adding within 100
- Addition and subtraction with number lines
- Add within 100 using a number line
- Subtract within 100 using a number line

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# Adding by making a group of 10

Sal adds 5+68 by breaking the 5 into a 2 and a 3.

## Want to join the conversation?

- im confused about this if you already know that the question is ex: 68 +32 why would you do all that instead of just doing the normal way.(50 votes)
- Yes, i agree. You may find something you missed. I will give you a vote for such a good answer.(12 votes)

- I'm doing a practice on "Add 2-digit numbers by making 10" and on this question says

"25 + 47 is the same as ___ + 50". And I used the hints and it said 3 ones from 25 to 47.

Where did the 3 ones come from?(8 votes)- The 3 ones come from the 25 and are added to the 47 to make 50.

So 25-3 = 22 and 47+3 = 50.

This makes the new sum 50+22 (or 50+22 as I would prefer) which is a sum that does not require carrying any 1's over (so its an easier sum).(9 votes)

- Jean add 35 and 9 .how can she solve using only equations to model her thinking ? explain(5 votes)
- You can broke it down to this much longer but clearer method. You have 3 tens and 5 ones (30 and 5 = 35), and you need to add it to 9 ones. So you can add first the similar numbers, like 5 and 9 which is in ones place value. Doing that would get you to, 14 ones, or 1 tens and 4 ones (10 and 4 = 14). From that 1 tens and 4 ones, you can apply again the previous method by adding first the tens, because they're similar. 3 tens + 1 tens (30 + 10) would give you 4 tens or 40 ones. Now you can add the remaining ones to the 40 ones (or 4 tens) which is 4 ones, and that would give you 40 tens and 4 ones, or 44 ones, or in number form 44.(3 votes)

- so like 50 PLUS 5 = 55(4 votes)
- Yes, because you have 5, and you add 0, which as a five next to it, it will be 55.(7 votes)

- I'm confused with 68+5. It seems much more obvious to just round 68 up to 70 by adding 2 that you borrow from the 5. Then add the remaining 3 to the 70 equaling 73. What am I missing? This seems easier and more logical!(6 votes)
- You can do it both ways, rounding or just use normal equations(4 votes)

- how do you do Adding by making a group of 10(3 votes)
- We are trying to add numbers by taking away from other numbers to make the process easier.

An example is that 4 + 18 is the same as 2 + 2 + 18

We're essentially breaking numbers to form groups of 10's.

2 + 18 is 20, and when you add the final 2, you get 22 (20 + 2)

20 + 2 is easier than adding 4 + 18 if you scale up to numbers you can't grasp.(8 votes)

- if it is 1,000x10,000 you take the the first digit of each number ad you multiply them so 1x1=1 the you add the the rest of the zeros so 1,000x10,000=10,000,000?(4 votes)
- Yes you are right. Multiplying the non-zeros and counting how many zeros to solve that kind of problem makes it faster.(4 votes)

- Hi, I am having some trouble with Adding by making a group of ten. Would anyone have a suggestion for me? I watched both videos, but I didn't get much out of them.(2 votes)
- Look at the bigger number, how much do you need to add to it to make a ten number (10, 20, 30, 40....)

You can take that amount out of the other number and add it to the bigger number. Now you can add them together more easily because the bigger number is a ten.

For example: 7+26

How much do I need to add to 26 to make 30?

2 6+*4 =*=**30**so I need to take 4 away from 7.

7 - 4**3**

So 30+3=33. It's the same answer as 7+26, but for some people it's easier to make then 10 than add random numbers together.(7 votes)

- Why does Sal include a number line at2:30to represent is process to tackle this problem when he could just do count to 73 from 68 to show that we added 5 to get there?(4 votes)
- What is which as a part of 1090 point me(2 votes)

- Does it matter if you break up the larger or smaller number while using this method?(1 vote)
- No, it does not matter which number you take apart.(7 votes)

## Video transcript

- [Voiceover] What I wanna show you now is a way to add numbers, or a way that I sometimes add numbers, when I'm doing it in my head. So let's say I wanna add five to 68. And we've already seen
other ways of doing it, so what I'm showing in this video isn't the only way to do it but it might be helpful when
you're doing it in your head. Well when I look at a number
like 68 I think well gee, If I added just two to
that I would get to 70. So why don't I break up
five into three and two, add the two to 68 and then
I have to add the three. So what's that going to get me? Well if I break up the
five into three plus two, and the whole reason why I
broke it up into three and two is so I have this two here to add to 68. So five is three plus two, and to that I'm going to add 68, 68. And once again the reason
why I took this two out is because I said hey, what do I have to add to 68 to get to 70? So now I could just rewrite this as, Three plus two plus 68, 68. But now I can add the two to the 68. So two plus 68 is going to be Two plus 68 is 70. 70 and I still have this three here, so I have three plus 70
which is equal to 73. 73. Now it might seem like this
really long way of doing it, but this is just a way to think about it. In your head you'd say okay five plus 68, let's see if I add two
to 68 I'll get to 70 and so let's see, five is three plus two, I add the two to 68, I get to 70, and I have three left over. So it's going to be 73. Another way you could think
about it is on a number line. So let's draw ourselves
a number line here. Let's draw a number line
and let me draw some let me draw some tick marks here. And let's say that this right over here, this right over here is 68, and so this is going to be 70, this is going to be 70, and we're gonna add five. We're gonna add five. So if you add five you say
well let me add two first. So if I add two first I get to 70. So plus two, that's what I did right
over here to get to 70, and then if I wanna add five
then I have to add three more. Then I have to add three more. And so it's going to be 70 plus three. Which of course is 73. Hopefully you found that interesting.