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Current time:0:00Total duration:4:57

CCSS.Math:

what we're going to do in this video is start trying to divide by two digit numbers and as we'll see this is a super important skill that a lot of the rest of mathematics will build off of but it's also interesting because it's a bit of an art so let's just start by trying to compute what 186 divided by 31 is and of course we could also write this as 186 divided by 31 pause the video and see if you can figure it out alright now let's work through this together I assume you've given it a go at it so I don't have my thirty-one times tables memorized so I can't just immediately tell you the answer but my brain tries to approximate these numbers so that my times tables knowledge does come in handy for example 31 is close to 30 this is why I'm saying it's a bit of an art but I'm saying hey look this is pretty close to 30 and if I think about multiples of 30 well those are just the multiples of 3 with an extra zero or the multiples of 3 times 10 so the multiples of 30 or 30-60-90 instead of 3 times 4 is 12 30 times 4 is 120 150 instead of 15 180 instead of 18 for 30 times 6 and gee this looks pretty close to 180 so this looks close to 30 times 6 and if this is close to 30 well maybe maybe this is maybe this is going to be 6 right over here and I'm going to put a question mark because then the natural thing to do is it to actually try it out figure out what is 6 times 31 I'll put the question mark here again is this equal to 6 well the art part is you've made a good or we've made a good guess here now let's try to verify that maybe it is 6 maybe it isn't so let's multiply 6 or let's multiply 31 times 6 31 times 6 1 times 6 is 6 and then 3 times 6 which is really 30 times 6 is going to be 180 right over here so this is 186 so it all works out 186 divided by 31 is indeed six we can get rid of this question mark here let's do another example so let me delete let me delete that as well let's say we want to figure out what 336 divided by 48 is equal to so pause this video and see if you can figure it out so the way I would think about it this thing right over here is close to 50 close to 50 and this thing well if you think about the multiples of 50 you have things like 300 which would be 50 times 6 and then you have 350 which would be 50 times 7 because 5 times 7 is 35 and so let's think about this and actually write those these things down if I were to say 50 times 6 that's the same thing as well that's going to be 5 times 6 times 10 which is 300 if I say 50 times 7 that's going to be 350 5 times 7 is 35 and you're multiplying that by 10 and this number is some place in between 300 and 350 so my candidates for what this is going to be well I'm saying hey maybe since 6 maybe this is 7 and what I would do is I would try out each of them so let's try out 48 times 6 so let's try that this first candidate out 48 times 6 so 8 times 6 is going to be 48 4 times 6 is 24 plus 4 is 288 so that actually seems a good bit lower than this it actually seems almost exactly 48 lower so I could probably I'm feeling pretty confident that I can squeeze another 48 into this so I'm going to get instead of squeezing 4 instead of being able to divide 6 48 into this I'm feeling pretty good that maybe 7 is the answer but let me try it out so this so 6 is not going to be my answer I tried it out let me try 48 times 748 times 7 8 times 7 is 56 4 times 7 is 28 plus 3 336 that's exactly right so there you have it with a little bit of trial and error but it was an informed trial and error we got 336 divided by 48 is equal to 7 this is what I meant but there's it's a bit of an art