Singapore Math: Grade 3a Unit 2 (part 2) Video Lecture | Mathematics Lesson for Grade 3 (Singapore) - Personal Learning

let's keep doing some of these Singapore
math problems so now on let's see what
am i I'm on unit 2 and I'm this is part
2 not officially part 2 but I just did
unit 2 part 1 so from my video point of
view its part 2 but the problem I'm
going to do is pop number 4 that's on
page 20 and it says it says Mary made
686 cookies
she sold she sold some of them some of
them if 298 were left over were left
over or how many cookies did she sell
how many cookies the hardest part about
this is writing it down did she sell
which you don't have to do because it's
already in your book how many cookies
did she sell did she sell maybe you
think that the hardest part about this
is reading my handwriting perhaps let's
see let's see if we can draw this out
well she made 686 cookies so let's let's
draw like a bar that represents all of
the cookies that she made so let's say
that this bar has length 686 were the
units or cookies instead of centimeters
well maybe that should be an
international standard of length the
cookie but anyway let's say that this
bar has a length of 686 and it
represents the total number of cookies
she made and that she says she sold some
of them we don't know exactly how many
she sold but let me see if I can so
let's say she sold some of them so the
number that she sold
let me just shade that in here let's say
this is how many she sold and we know
that after she sold a bunch she had two
hundred and ninety eight were left so
298 were left
the 298 and if I'm just approximately
that's about 300 so it's a little bit
less than half so roughly this much was
left 298 so that's the 298 shade that in
right there so this length is 298 so the
question is asking us how many cookies
did she sell well the 298 is how many
she had left over and so what she sold
must be this blue chunk this is what she
sold right and that should make
hopefully makes a little bit of
intuitive sense for you if you make 686
and you sold 298 or sorry if you make
686 and you're left with 298 the amount
that you sell must be the difference
between 686 and 298 and so what is the
difference or this blue region between
686 and 298 well we learned in the
previous video the badge is just equal
to 686 minus 298 right this whole thing
- this is equal to the amount that she
sold is equal to and you could do that
math and and actually what I'm gonna do
here and I'm assuming that this is
covered in Singapore math grades
probably grade two but I'm actually
going to review it just because I think
it's interesting and frankly I think you
know once you know how to do subtraction
and regrouping with three did it's it's
pretty much the same thing to do with
four or five or six or seven which will
do shortly but I just want to make sure
you really understand the intuition here
so if we can figure out what 686 minus
298 is we have solved the problem we
have figured out exactly how many
cookies Mary sold so let's see if we can
do that so let's figure out what 686 686
minus 298 is and now I'm going to go
back to what we did with all of the
places and you know and and our buckets
of Baral of marbles and all of that
because I really want to give you an
intuition of what
when we subtract these two numbers or
what's happening when we when we do
things like borrowing or regrouping so
let's say both of these are three-digit
numbers we have our ones place our
tenths place in our hundreds place so
let's draw those columns and I forgot
the colors that I had used previously so
let's just say the hundreds place this
is hundreds this is tens and this is
once let me draw some dividing lines so
we're starting off with six hundred and
eighty six in the case of this problem
it's six hundred eighty six cookies
right so what does this six represent
this six essentially represents six once
right so we could draw them here one two
three four five six and these are all
ones what is this eight represent well
those are tens right so we have eight
tens
one two three four five six seven eight
and each of these represent ten cookies
or ten things whatever we're measuring
these are tens and what do what does
this six represent well that's six
hundreds right so if we look at a
hundred place we could view this may be
the top of cans one two three four five
six and each of these represents a
hundred each of these represent one
hundred and we're going to take away two
hundred ninety eight right six hundred
eighty six minus 298 so how do we do
that
well our first temptation might be well
let's just take away eight once right
but we don't have six ones here I mean
we don't have eight once you really have
six one two three four five six so how
can we take away eight ones well what we
could do is we could take we could take
one of these tens and turn it into
once so what we could do is let's list
let's take this ten and convert it to
ten ones right all we're doing is is you
know we're taking we're opening up this
can of marbles maybe when we're using
cookies in this example instead of
having a can of 10 we're gonna take out
all the marbles and put them as one so
let's let me do that so I'm gonna cross
this out here and I'm gonna put ten
marbles here or 10 cookies 1 2 3 4 5 6 7
8 9 10 and all of these are 1 and where
did I get them from where remember I'm
just trying to started line to show that
I got them by exchanging this right here
so how many ones do I have now I have 10
plus 6 so I have 16 ones now so now I
can take away 8 of these right I could
take away so I see let me write it this
way so I had 686 let me figure out some
space where I can write this neatly so I
had 686 no I don't have space minus 298
so what did I just do I took away one of
these eight tens so now I only have
seven left right and I converted it to
10 ones so now I have 16 once so those
are view familiar with borrowing I
borrowed one from the eight or we could
say we regrouped we took 10 and we put
it into the ones group but either way I
hopefully all of this will make sense to
you because these are all just different
ways of doing the same thing and now we
can take eight away we can't we have 16
once we can take 8 away so what happens
when we take 8 away 1 2 3 4 5 6 7 8 and
then how many do we have left we have 1
2 3 4 5 6 7 8 left so we have 8 left
I'm sorry for writing this so messy
actually let me clean this up a little
bit this is ridiculous
that's my tens I want to free up some
space so I can do this relatively neatly
let me change this let me make that line
so this is dense it's just left some
space so that's dance okay so what did
we do
we borrowed 10 from the tens place or we
could say we cashed in this 10 bucket
and we got 10 ones here and that's where
the six became 16 and then once since we
used one of these tens we now have seven
left in the tens place fair enough
it's sixteen minus eight is eight right
we at sixteen we got rid of 80 of eight
but now we have seven tens right one two
three four five six seven and that's
represented by seven here but we want to
subtract out nine of them so once again
we don't have enough tens so what can we
do well we can do the same I don't wanna
call it a trick because it's nothing you
know tricky about it we're just we could
take in we could cash in one of these
and get ten tens for them so let me do
that so let's take this one and convert
it to ten ten so we have one two three
four five six seven eight nine ten and
each of these are ten each of those are
ten and we cashed that out right so now
what is our equation what is our not
equation what is our our problem look
like well how many tens do we have now
we have well let me the how many hunters
do we have we have one two three four
five right we used to have six now we
have five and that makes sense because
we took one of them we took one of them
and we converted it into tens and so how
many tens do we have now we have one two
three four five six seven eight nine ten
11 12 13 14 15 16 17 so we have 17 well
let me just put a 1 here I should cross
out of 7 but we could just put a 1 here
so we have 17 tens right so now and you
know if you're familiar with the term
borrowing that's essentially what we did
but we didn't really borrow right we
don't we're not gonna it's not like
we're gonna give it back that's barring
we took it we took it from the hundreds
place so we took this this 100 canister
of cookies and converted to 10 canisters
of 10 cookies
that's what we did and so that's why our
six canisters of 100 turned into five
canisters but that's why we got 10 extra
10 cookies in the tens place hope I'm
not confusing you but anyway now we can
subtract right we have to take out nine
of the tens right this is the tens place
so we have 17 let's take out nine so one
two three four five six seven eight nine
and so how many do we have left we have
one two three four five six seven eight
so we have eight left we have eight left
oh that may be you know 17 minus 9 is 8
fair enough now we go to the hundreds
place we have five hundreds left we had
six but we had to convert one of them
into into tens so we have five of those
but then we have to take out two
hundreds so let's do that
one two and how many do we have left
with one two three three hundreds left
so 686 minus 298 is 388 and you can see
that here one two three 380 this is one
of them two one two three four five six
seven eight 388 one two three four five
six seven eight so hopefully that wasn't
confusing to you I mean you might have
already known how to how to borrow or
every group like this but this should
have hopefully given you a little bit
intuition but just remember all
everything I did just now now is to
solve that original problem where Mary
had made 686 cookies she sold some of
them and she had 298 left so essentially
we now know that she must have sold 388
cookies and he could view it the other
way you could say she had she started
with 686 cookies
she sold 388 cookies and now she has 298
left and you could do the subtraction
confirm that it's true and this backs up
that you know the difference between 686
and 388 is 298 therefore mean 686 and
298 is 388 anyway I'll see you in the
next video