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# Breaking apart 2-digit addition problems

CCSS.Math:

## Video transcript

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 30 plus 60 plus 90 plus 10 and I encourage you to pause the video and try to work this out actually before I do it so this first choice so where did they get this 30 from well 30 that's three tens three tens and I do have three tens right over here the three and 39 that's in the tens place so it represents three tens or thirty and then we have 60 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 well I don't see any obvious 90 over here it might be tempting to say well I have a nine over here but this isn't the ones place it's not in the tens place this is nine not not 90 and over here I have one I definitely don't have a 90 or I definitely don't even have a 10 so this would make sense instead of its this didn't say 90 if this said nine instead of a ten if this said if this just said one because we have a 1 in the ones place then it would make sense but it didn't say that it didn't say 30 plus 60 plus 9 plus 1 is saying 30 plus 60 plus 90 plus 10 so we're not going to pick not going to pick this choice the next choice we have 30 plus 60 plus 9 plus 1 which makes complete sense because because we have the 30 plus the 9 plus the 9 is going to be equal to 39 and then the sixty the sixty plus the 1 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 because you can compute in your head 30 plus 60 that's three tens plus six tens is going to 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 going to 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 is 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 going to 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 thirty this should be thirty and this is six tens not just six so that should be sixty but that's not what they 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 forty one 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 let's see which of these choices are the same is what I just wrote here so this first one we have four tens four tens and one one four tens and one one four tens and one one that's forty one what I just so this four tens plus one one would be forty one and then I also have five tens and 7 ones five tens and 7 ones so this first choice is exactly what I wrote down it's just in a different it's just in a different order if I write four tens plus five tens plus one one plus seven ones it's going to be this first choice and we know that we're done but wait let me just look at the other choices just see why these don't make sense so I see where the for tens come from but then it has one ten it looks like it's trying to take this one in a ones place and somehow turn it into one ten so that's definitely going to be wrong and then it says five one somehow this is five tens here five ones so that's 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