Breaking apart 2-digit addition problems | Addition and subtraction | 1st grade | Khan Academy

• Let's think about ways to break up addition problems. And this is useful because if we break them up in the right way, it might be easier for us to actually compute the addition. So let's look at this first question.

Lindsay isn't sure how to add 39 plus 61. Help Lindsay by choosing an addition problem that is the same as 39 plus 61. So let's look at these choices. This first choice I have is 30 plus 60 plus 90 plus ten. And I encourage you to pause the video and try to work this out actually before I do it.

So this first choice, where do they get this 30 from? Well, 30, that's three tens, and I do have three tens right over here. The three in 39, that's in the tens place, so it represents three tens or 30. And then we have 60, where did that come from? Well, in the number 61, the six is in the tens place, so it represents six tens or 60.

And then we have plus 90. Now where is 90 coming from? I don't see the obvious 90 over here. It might be tempting to say, well I have a nine over here, but this is in the ones place; it's not in the tens place. This is nine, not 90. And over here, I have one. I definitely don't have a 90. Or I definitely don't even have a ten. So this would make sense, instead of if this didn't say 90, if this said nine, and instead of a ten, if this said one, 'cause we have a one in the ones place, then it would make sense, but it didn't say that.

It didn't say 30 plus 60 plus nine plus one; it's saying 30 plus 60 plus 90 plus ten. So we're not gonna pick this choice. The next choice, we have 30 plus 60 plus nine plus one, which makes complete sense because we have the 30 plus the nine, is going to be equal to 39. And then the 60 plus the one is going to be equal to 61. So these two things are equivalent.

And the reason why it's useful to break up things this way is 'cause you can compute in your head. 30 plus 60, that's three tens plus six tens is gonna be nine tens. So these two pieces right over here are going to be, that's going to be 90. And then nine plus one, that's gonna be ten. And then 90 plus ten, well that's going to be equal to 100.

Now, they didn't ask us to compute that; they're just saying, "Hey, which of these are the same as what we have up above?" and this one is definitely going to be the case. And we can only pick one here, so we're done, but we can verify that this one isn't gonna be right. We have the nine plus one, then they have three and six.

Now this three here, that doesn't represent just three; that's three tens, that's 30. This should be 30. And this is six tens, not just six, so that should be 60, but that's not what they originally wrote. So we can rule that one out as well.

Let's do another one. Which addition problem is the same as 41 plus 57? And here they broke up everything into, looks like tens and ones. So even before I look at the choices, let me see if I can do that.

So 41, in the tens place I have four. So that's going to be four tens, and in the ones place I have one. Plus one one. That's 41. And then 57, 57 in the tens place, I have five, so plus five tens, and in the ones place, I have seven. Plus seven ones. So let's see which of these choices is the same as what I just wrote here.

So this first one, we have four tens and one one. Four tens and one one. Four tens and one one, that's 41, what I just... so this four tens plus one one would be 41. And then I also have five tens and seven ones. Five tens and seven ones.

So this first choice is exactly what I wrote down; it's just in a different order. If I write, four tens plus five tens plus one one plus seven ones, it's gonna be this first choice. And we know that we're done, but let's just look at the other choices to see why these don't make sense.

So I see where the four tens come from, but then it has one ten. It looks like it's trying to take this one in the ones place and somehow turn it into a one ten, so that's definitely gonna be wrong. And then it says five ones; somehow this is five tens here, not five ones. So that doesn't make sense.

And then this one says four ones; well this four, this is in the tens place, this is four tens. And then we have five ones, that five is in the tens place. It should be five tens. That should be tens, and that should be tens. So we feel good about the first choice.