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# Missing numbers in addition and subtraction

CCSS.Math:

## Video transcript

let's say someone walk walks up to you on the streets as quick 73 plus blank is equal to 57 what would blank be well there's a couple of ways to think about it blank is essentially what you have to add to 57 to get to 73 it's the difference between 57 and 73 and so you could think about it as well let me start with 57 and this is actually how I would do it in my head I would start with 57 and I would add numbers that get me two nice round numbers until I get to 73 so I'd start with 57 I would say let's see I could get to 60 pretty easily by adding three so that's going to get me to 60 and from 60 to 70 three well I can imagine I'm going to add 13 that's easy for my brain to compute or I could say I'm going to add 10 to get to 70 and then I could add another three but my brain could say sixty plus thirteen yeah that's going to be seventy three so now this whole thing is 73 so what did I add 257 to actually bridge the gap I added three I added three plus thirteen so I added 16 so 16 plus 57 is equal to 73 now there's other ways that you could try to tackle it you could say looks this blank is a difference between 73 and 57 so you could write this as 73-57 minus 57 is equal to blank is equal to blank and this would get you the same value would get you it would get you 16 and there's multiple ways to compute this but this is the way that I would actually try to tackle this in my brain let's do a few more examples this is strangely strangely fun so let's say that someone another person walks up to you on the street and says quick 94 - blank 94 - blank is equal to 57 is let me write this in that color is equal to is equal to 57 so this is saying what do I have to subtract from 94 to get to 57 well I could do it in a similar way I'm subtracting this time where I could keep subtracting numbers that make the math easy in my head until I get to 57 and this is see how I would do it in my head I would start at 94 and then I would subtract for to get to 90 so that's 90 right over there and let's see then I could subtract I could subtract 20 to get to 60 I don't want to get I don't want to get lower than 57 so I'm going to subtract 29 minus 20 oh sorry I can subtract 30 to get to 60 so I could subtract 30 now so 90 minus 30 this is going to be equal to 60 and then to go from 60 to 57 pretty straightforward I just have to subtract another 360 minus 3 this is going to be 57 so how much did I subtract I subtracted for 30 and 3 so let's see I subtracted this is 34 37 I subtracted 37 and just like that now another way you could have done it you could have said look 37 is the difference between 94 and 57 so you could have thought about it well if I start at 57 what do I have to add what do I have to add to get to to get to 94 to get to 94 and you could have tackled it exactly the way that we did the last problem where you could do a I'm going to add 3 and then I'm going to have to add third if I had 3 I get to 60 then if I had 30 I get to 90 and then if add another 4 I get to 94 so I added 37 so I added 37 now there are clearly other ways to do this especially if you have especially if you have paper around but this is actually how my brain would tackle it if someone walked up to me in the street and asked me to solve these problems now let's do one more let's do one more of these let's see where I have a clear space so let's say that someone asks you have 36 is equal to blank is equal to blank minus 41 blank minus 41 so the way I think about this is they're saying that 36 is the difference between blank and 41 or another way to think about it is 36 plus 41 36 plus 41 plus 41 is going to be equal to blank is going to be equal to blank you could draw this on a number line if this if these two statements being the same doesn't make full sense if we do a number line right over here and then they're saying okay we're going to start at some mystery number we're starting at some mystery number that's our blank we're going to subtract 41 we're subtracting 41 so we're subtracting 41 and we're getting to 36 and we're getting 2 we're getting to 36 so this is the exact same thing as saying if we start at 36 and if we were to add 41 if we were to add 41 we get back to a blank right over here so what's 36 plus 41 well we could add we could add our tenth let's add our ones first so we have six ones plus one one that is seven ones and then we have three tens plus four tens which is seven tens 77 so this is 77 and we're done