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Current time:0:00Total duration:3:30

CCSS.Math:

we're told that Linda isn't sure how to subtract three 853 we are told Linda isn't sure how to subtract 853 minus 283 help Lindy by choosing expression that is the same as 853 minus 283 so pause this video and see if you could answer it on your own before we work through it together all right now when we look at all of the choices they all start with 853 now this first choice they subtract out 200 that makes sense because we have two hundredths right over there then they subtract out fifty and then they subtract out three so the two hundred and the three makes sense you can view 853 minus 283 as 853 minus two hundredths minus 88 tens right here minus three but that's not what they wrote over here instead of putting an 80 here they put a fifty so we can rule that out here we have 253 minus twenty minus eight hundred minus three well this is a little bit strange because we don't have two tens we have two hundreds and we don't have eight hundreds we have eight tens so this is also incorrect now it's probably going to be this one but let's just make sure we feel comfortable with this so this one has eight hundred fifty-three minus two hundred minus 50 minus thirty does that make sense well let's think about it what they're doing is that they're subtracting out first the two hundred so that's this part right over here so that makes sense they're subtracting out first that and then they're subtracting out 53 and then the subtracting out 30 well that is the same thing as subtracting out 83 because subtracting 83 is the same thing as subtracting out 53 and then subtracting 30 53 plus 30 is 83 now you might be wondering why would they even do it this way well it's easier to do in your head 853 minus 200 is six hundred and fifty three you take away 53 from that so you take away 53 from that and you're going to be left with 600 and then 600 minus 30 you might recognize that you could do this to 60 tens minus three tens there's going to be 57 tens or you might be able to do it in your head 600 minus 30 is 570 so that's why they broke out they broke up the 83 instead of just breaking it up into 80 and 3 they broke it up into 53 and 30 to match this 53 over here and so it was easier to work out the result let's do another example so here we're asked to fill in the blank they say 143 minus 79 is the same as blank minus 80 so pause this video and see if you can figure this out so the key here is to realize is that when you have a difference here as long as you add or subtract the same amount to both of these the difference will be the same so it looks like they tried to turn the 79 in two it looks like they turned the 79 into an 80 by adding one and so if we want the difference to be the same we would add one to the hundred and forty three as well so 143 plus one is 144 if we add one to both of these numbers the difference would be the same and we're done so it's 144 minus 80