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Current time:0:00Total duration:3:58

Video transcript

clothes do some practice problems on Khan Academy exercises that make us rewrite an addition problem so that we can get them to rounder numbers numbers that are might be multiples of 10 or num multiples or multiples of 100 let's see here I have 63 plus 427 and that seems kind of kind of hairy you know I feels like I want to write those things down but maybe I could take away from one of them and give it to the other one so that they both become round numbers and it's clear that 63 if I were to give three away I would get 260 and if I were to give those three to four hundred and twenty-seven it would get to four hundred and thirty and sixty plus four hundred thirty is a much simpler problem so let's think about what this question is saying well it's just taking a step by step through that process it's just saying 63 plus 427 is going to be equal to 60 Plus what plus 427 so the 60 plus what this is going to be the same thing as 63 because we have 427 both places so 63 is 60 plus three all that makes sense now in this next step they just changed the order 60 plus three and then adding 427 is the same thing as doing 60 plus and then adding the 3 to the 427 first so we're just taking this 3 and moving it from the 63 to the 427 well 3 plus 427 that's just 430 and now this addition problem became a lot simpler 60 plus 430 we could do that in our head we're just adding 6 tens to this right over here so it's going to be 400 490 and we're done let's do a couple more examples of this so over here we want to let's see we want to add these two numbers and let's maybe see if we can maybe we can make them a little bit of rounder number so over here we're breaking up the 275 into 270 and something well that's going to be 270 plus 5 notice the rest is still the same plus five ninety five plus five 95 now why are we doing that well if we take five from 275 and give it to 595 which is what we're doing here the 595 can get to 600 it's going to make the calculation easier so once again here we're adding the 270 in the five first and then we're adding to 595 but we could change the which we do it we could add the 5 to the 595 first and then add the 270 so this is the same thing as 270 plus 5 plus 595 is 600 that's the whole reason why we why we took five from the 275 is so that we can add it to the 595 and get 600 and now we can do this in our head 270 plus 6 hundredths well we're just going to increase our hundreds by six so it's going to be eight hundred and seventy let's do a couple more couple more examples here fill in the blank 51 plus eighty three is the same thing as blank plus 84 well they increased 83 by one to 84 so we've got to decrease 51 by one so this is going to be the same thing as 50 plus 84 now why would anyone care about this why would they do that well I find this easier to calculate because now I just have to say eight tens plus four ones plus another five tens well that's going to be 13 tens and four ones or 134 so I find this a little bit a little bit easier but the important thing is if we add to one number we got to take away the same amount from the other number in order to not change the value of this of this addition problem of this expression let's do one more 138 plus 7 or 10 is the same as blank plus 700 so we had 7/10 and it's now 700 so we took 10 away from that number so we have to give it to the other number so 138 we have to add 10 to that so that's going to be 148 now why was that useful well 148 plus 700 you can do that in your head that's going to be just going to be 848 and that's easier to compute than what we just had over here