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### Course: 1st grade>Unit 2

Lesson 9: Intro to addition with 2-digit numbers

# Regrouping to add 1-digit number

Sal adds 35 + 6.

## Want to join the conversation?

• How can you train your brain to see arithmetic this way, after thinking of it differently your whole life?
(48 votes)
• When you do the exercises, apply the concepts as Sal has done in the video. Go slowly at first if you need to, but you'll see how quickly these concepts become second nature with some practice. Gook luck!
(11 votes)
• Ok I have a problem. Essentially (and this did come up in a question) this teaches that 35 + 6 = 30 + 11

Let's apply this to a real world problem. A man who already has 35 scoops of ice cream purchases 6 more scoops and gets a bill.
A man who already has 30 scoops of ice cream purchases 11 more and gets a bill.
Assuming both pay the exact same price per scoop who pays the larger bill?
The guy who is about to buy less scoops does. My question is does 35 + 6 truly equal 30 + 11? The answer from both equations is the same but does an equal answer dictate an equal equation?
(4 votes)
• You are forgetting that the guy who bought 35 scoops has already paid more than the guy who bought 30 scoops. So even though he pays less the second time, he paid more the first time. If they both buy 41 scoops, and each pay the same price per scoop, then their bills would be the same after all of the scoops are purchased.
(5 votes)

## Video transcript

So, we have the number 35. The 3 is in the tens place so it represents 30 or 3 tens— one 10, two groups of 10, three groups of 10 and then the 5 is in the ones place so it represents five ones. We see them right over here —one, two, three, four, five. Now, we want to take that 35 or those 3 tens and 5 ones and add 6. 6 ones. The 6 is in the ones place— one, two three, four, five, six ones. And I encourage you to pause the video and try to do that. Add 35 to 6. So let's think about it now. So I'm gonna start with the ones. So I have 5 ones and I want to add 6 ones. So what's that going to be altogether? Well, 5 ones plus 6 ones, that's going to be 11 ones and I still have 3 tens so we could say it's going to be 3 tens, 3 tens and 11 ones. 3 tens, I could write, plus 11 ones here. Now this is a little bit of a problem because we can't write a two-digit number in the ones place or in any one of the places. I can't, this number isn't going to be 311. It's going to be 3 tens and 11 ones but how can I rewrite this or how can I regroup things so that I only have a single-digit number here? I have zero, one, two, three, four, five, six, seven, eight, or nine here instead of a two-digit number— the 1 one. Well, I can regroup. I can say, "Look, I have enough ones here to create a group of ten." I could take 10 of them, so let's take these 10, right over here, and put them together and make a new group of ten. So, a new group of ten right over there. So if I take, so just to be clear, what I just did I just took these 10, these 10 ones and I stuck them together and I turned it into this new group of, this new group of ten. So now what do we have? So when you regroup like this you see that you have one, two, three, four tens. 4 tens. And how many ones do you have now? 4 tens plus? Well, I've regrouped all of these 10 ones and all I have left is this 1 one right over there. So I could write this as 4 tens plus 1 one. Once again, 3 tens and 11 ones, that's the same thing as 4 tens and 1 one. And so we can write that over here, we can write this as, 1 one and 4 tens. Now how could you get this if you weren't, if you didn't do it like this and drawing everything out and regrouping like this and this is actually what you should be doing in your head but another way of thinking about it, you could say, "Alright 5 plus 6," that's going to be 11 ones but I can't write an 11 here in the ones place so I could say, "That's going to be the same thing as 1 ten plus 1 one." 1 ten plus 1 one. Sometimes it's taught that 5 plus 6 is 11, carry the 1 but really what you're doing is you're saying, "5 plus 6 is 1 ten plus 1 one." And, in fact, an 11, the number 11 right over here. So if I were to write the number 11, the number 11 has a 1 in the tens place and a 1 in the ones place so it's 1 ten plus 1 one. So you're just saying that 5 plus 6 is 11 which is the same thing as 1 ten and 1 one and then you add your tens together— 1 ten plus another 3 tens is going to be 4 tens. But I really want you to appreciate what's going on. You're not just blindly saying, "Oh, 5 plus 6 is 11 so I'm going to write 1 of the ones here and write the 1 in the tens place here." You're doing it because you're regrouping. You're regrouping that group of ten. You're saying, "Hey I could take 10 of these ones and I can turn them into a new group of ten and that would just leave me 1 in the ones place. 1 in the ones place and I've just turned all the other ones into a new group of ten.