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## Arithmetic (all content)

### Course: 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?

- 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?(12 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).(13 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.(7 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.(14 votes)

- This is a bit difficult to swallow. It seems like it is just a way to complicate a relatively simple problem, but on the other hand it could help with larger numbers. The number line towards the end seems more helpful in remembering that the larger the number the further right one must travel on the line.

I think the answer would change depending on how you group the numbers in the parentheses too. Is that right?(5 votes)- Sal uses two equations: (3+2)+68 and 3+(2+68). The Associative Property of Addition states that these equations are equal. Let's work them both out starting with (3+2)+68.

(3+2)+68=

5+68=

73

Now let's solve 3+(2+68).

3+(2+68)=

3+70=

73

Hope this helped!(5 votes)

- At0:57what do those things mean (3+2)?(2 votes)
- What those ( )'s are at0:57are known as brackets or parentheses. These are used alot in more complex equations to seperate the different terms.(4 votes)

- Why can't you add 8+5 then 60+ the answer for 8+5 (which is 13) wouldn't it be easier?? I really don't get it could someone give me an opinion or answer??(3 votes)
- For some people it might be harder and for some people it might be easier. For example, I would split the five into 2 + 3 and add the 2 to 68 to get 70 and then add the 3 to get 73, but some people think that's harder. It really depends on what you think is easiest. So you can definitely do that if you want!(4 votes)

- but what happens if you get a question like 47 ➕ 23 now what?(3 votes)
- There are multiple methods for
**2 digit (a number with two numbers in it) addition**. E.g, the column method, a relatively easy one:

Think of it like this, you put the numbers into two columns:

1) 47

+23

*___*

*___*

The first step you do, is**add the two numbers in the first column from the right side: 7 + 3, that equals to 10**.

2) 47

+23

*___*

*__*0**

But**10 is a two digit number**, we don't have any space to fit both digits. So we put**the first digit from the left in that column, 0, and then**:

3) 47

+23

*___*

*__*0**

**1****We carry the 1, from the 10, under the next column**.**It will help us find the answer but it is not part of the answer**.

4) We now do**the second column**:

47

+23

*___*

*__*0**

**1**

We**add 4 + 2 = 6**, there just one digit, 6 BUT

Remember,**we have a 1 underneath that column**, so we**add that 1 to 6, 1 + 6 = 7**.**We add all the numbers in that column, 1 is in that column**.

5) 47

+23

*____*

*7_0_*

̷*1̷*

We can cross out the 1 because we have already used it. Therefore, 47 + 23 = 70.

Remember to check that you've added all the numbers in each column. (I can't bold the text for some reason D:)

Here's another example:

1) 56

+90

*___*

*___*

2) 56

+90

*___*

*__6*(6 + 0 = 0)

3) 56

+90

*___*

14_6 _ (5 + 9 = 14, we put the whole number in, because we don't have other columns or any other numbers to add.

56 + 90 = 146

I hope you get the general grasp of this method :) - I'm not good at explaining, have a nice day!(3 votes)

- I would never think of breaking up a 5 to 3 plus 2. Why are these steps necessary?(2 votes)
- Kathryn you honestly don't have to do that. Its all up to how you learn. Sal is simply using one of HIS ways. (Remember these videos are for younger kids) So, he's taking it slow. A lot of kids learn like that. They learn one way, and continue using that way until they either get past that level, or they discover a different way.(2 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.