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Current time:0:00Total duration:4:26

CC Math: 4.NBT.B.5

- [Instructor] In a previous video, we figured out a way to
multiply a two-digit number times one-digit number. What we did is we broke
up the two-digit numbers in terms of its place value,
so the three here in the tenths place that's three tens,
this is seven ones. So we view 37 sixes as the
same thing as 30 sixes, three tens times six plus
seven sixes, seven times six. And then we added those
together to get a total of 222. What we'll try in this video
is now see what happens if we try to do two digits times
two digits, so let's try to tackle 37, 37 instead of six
let's multiply that times 26 so we're trying to figure
out 37 26's is one way to think about it. So pause this video and
see if you can tackle that and see if you can maybe
use a similar method to what we used before. Well one way to think about
it is you could view this as 37 26's or you could view
this as 37 sixes plus 37 20's. So first we could do the 37
sixes which is exactly what we did over here, we said hey
that's the same thing and we could do it in either order
we could say hey let's first think about 30 sixes, so we're
going to multiply 30 times six 30 sixes is 180, so that
right over there is three tens times six. And then we could think about
the seven ones times six. And so that's going to be 42. That's the seven ones times
six or the seven sixes. And then we could do the same
thing with the 20's we could say hey what are three tens,
what are 30 20's going to be. So let's write that over here
or you could say what are three tens times two tens. Well that would be six times
ten times ten, so let me write this down this is three tens
times two tens that's what we're going to do now. Times two tens and that's
the same thing as which is equal to 30 times 20 which
is the same thing as three times two times ten times ten. Well that's going to be 600,
so we could write that here 600 and just be very clear,
we've already thought about 37 sixes, that's these two
numbers up here we have to add that we still have to add them. But now we're thinking about
37 20's so first we thought about 30 20's which is 600
and now we could think about seven 20's so seven 20's is
going to be seven times two is 14 so seven times 20 is 140. So I'll write that right over here. 140 to be clear this is
seven ones times two tens. Or seven 20's and now we
can add it all together to get what the total would be. So in the total we have,
that's why it's useful to have everything stacked by their
place value, we could look at the ones place so we say
okay we only have a total of two ones here, so I'll put a two there. Now let's see tens we have
eight tens plus four tens is 12 tens plus another
four tens is 16 tens. 16 tens we can also break up
as 106 tens, so we could write the six tens here and
then put that 100 up here. So 100 plus another hundred
is 200 plus 600's is 800's plus one more hundred is 900's. And so there you have
it, this is equal to 962. I really want you to
understand what we just did, it might look a little bit
complicated but first we thought about what is 37 sixes that's
where we got these numbers from and that's what we had
done in a previous video. And then we just thought
about well what is 37 20's going to be and that's where
these numbers came from and actually let me write that down. This whole thing that I'm
circling in orange, that is 37 sixes or I'll write it as 37
times six and then this is 37 20's so 37 times 20. So if it's not obvious, pause
the video or after this video reflect on why this works,
try it with other numbers 'cause if you really
understand this then your multiplication life and
actually your mathematical lives in your future will only
become more and more intuitive.