Singapore Math: Grade 3a Unit 2 (part 9) Video Lecture | Mathematics Lesson for Grade 3 (Singapore) - Personal Learning

let's do some more subtraction problems
so let's say I had I don't know well
actually I do know because I'm looking
at the problem in the Singapore math
book it says 6000 let me do it a new
color
so six thousand and four minus two
thousand six hundred and seventy-eight
and I'm going to do it with the Sal
expansion but it's really the same thing
as when we you know do the Singapore
Math place value buckets with the with
the with the little circles with that
tell us how much of each place we have
because if we expand this out six
thousand four that equals six thousand
plus zero hundreds plus zero tens plus
four mm actually let me edit undo undo
that's gonna take forever
well anyway plus four I wanted to add
add some extra space here because I
realized in the last problem that's what
the problem was and I'm going to
subtract from it two thousand plus 600
plus seventy plus eight so let's do the
problem two is less than six that's cool
six is not less than zero so we have to
do something there seven is not less
than zero eight is not less than four so
we're gonna have to do some regrouping
so let's take a thousand from here from
the thousands place so all we're left is
with 5,000 and let's put that thousand
in the hundreds place so we'll have a
thousand here and how does that look
here well we'll have five thousand left
and now we're going to have ten hundreds
ten hundreds is the same thing as a
thousand well so now these two I guess
we say columns look alright but the
seventy is still greater than the zero
so what can we do well let's take a
hundred from this thousand and turn into
ten hundreds or to enter into 100 so
let's take 100 from here so we'll have
900 left and then put that hundred here
so that looks good but then we have an
eight as being less than a four I'm
sorry eight and eight being greater than
a four I just had some dark chocolate
and I apologize if
if you hear some pauses in my voice
anyway so how do we fix that well the
same way we've done all of everything
else let's take ten from this place and
put it into ones place if we take ten
from 100 we get 90 and we're gonna put
the 10 here so that becomes 14 oh I
forgot to do it on this side right so we
took a thousand from the 6,000 we have
5000 left and we have 10 hundreds or a
thousand then well we needed we took a
hundred from there so let's take a
hundred out of ten hundred we have 900
left and then we put we make and then
one hundred becomes ten tens but then we
need to borrow one of those not borrow
take one of those ten tens and and put
it into the ones place so we have 4 plus
10 is 14 so these two things are
equivalent and now we are ready to
subtract that's the hard part this is
the fun part now 14 minus 8 is 6 90-70
is 2900 minus 600 is 305,000 - 2000 is
3000 or 400 here 14 minus 8 is 6 9 minus
7 is 2 which is really the 20 here 9
minus 6 is 3 this 3 is actually this 300
and then 5 minus 2 is 3 which is really
this 3000 right because it's in the
thousands place let's do some more and
maybe I won't expand it out but I'll
talk it through ok I have four thousand
minus 392 so we immediately see that
these zeros are not going to help us
because they are less than the number
below it so what can we do let's take a
thousand from the 4000 we have so that
means we have 3000 left I'll switch
colors
we took a thousand and but if you put a
thousand in the hundreds place it
becomes ten hundreds right so these two
columns look alright but now we need
some tens so we can subtract here so let
me take one of the hundred we have ten
hundreds right let me take one of them
so we have nine hundreds left and make
this and then if we 100 is 10 tens so
that looks ok but let's take one of but
then we need to borrow for the ones
place so let's take one of those 10 tens
so it's a nine we have nine tens left
and then we add that 10 here and now we
are ready to subtract 10 minus 2 is 8 9
minus 9 or 90 minus 90 is 0 900 minus
300 is six hundred and three thousand -
nothing is three thousand let's do
another one
three thousand and twenty
- two thousand four hundred and thirty
well this column works out zero minus
zero that we can do that but this 3 is
greater than this - this 4 is greater
than zero so we have to do something
similar so let's take one of the
thousands so we're gonna have to left
and make ten hundreds out of it then we
need to make these the ten space bigger
so let's take one of our hundreds so we
only have nine left and at and turn that
into ten tens so this 2 becomes two tens
become 12 tens and we're all set now you
can subtract 0 minus 0 is 0 12 minus 3
is 9 9 minus 4 is 5 & 2 minus 2 is 0 so
we don't have to write it so our answer
is 590 let's keep going I'm just gonna
keep going until I use up 15 minutes and
now I'm actually going to write it out
with the Sal expansion style just
because I figure it doesn't hurt to do
so because I really want you to
understand what you're doing cuz if you
know what you're doing
you'll never get confused you'll never
say oh my god what was the next step I
forgot what the next step was because
they won't just be steps to you they
will be actual sense-making
operations five thousand minus two
thousand and seventy four and let me
write the expansion that equals
five thousand plus zero hundreds plus
zero tens plus C or ones and the bottom
number is two thousand plus zero
hundreds plus the 70 plus four okay so
what's the situation here
well this column is fine my phone
ringing that's my mom I'll talk to her
later this column is fine two is less
than five zero is at least equal to zero
so we could subtract there but these
these two columns are giving a problem
right and what's interesting here you
might say okay I don't have enough tens
here so let me take something from the
hundreds but you're like wait I don't
have anything in the hundreds place so
what am I going to do so this is what
you're going to do you have nothing in
the hundreds place but you do have a lot
of thousands so what we're gonna do is
we're gonna borrow from the thousands
place so let's do that so if we we're
not borrow we're going to take from the
thousands place so we're gonna take a
thousand so we're gonna have four
thousand left we're gonna take a
thousand have four thousand left so we
have a thousand to play with right and
what we can do is well just for
simplicity let's take all of that
thousand and write it in the tens place
now I'm sorry in the hundreds place so
if I we took a thousand from the
thousands place and let's put it into
the hundreds place so in the hundreds
place would be represented as ten
hundred right so we took a thousand so
we can write that thousand right here
that's ten hundred that's why it's a ten
here and that's why certain it's a
thousand here but we this isn't where we
need it we need it over here these areas
so let's take out of this thousand let's
take a hundred to put in the tens place
so let's take a hundred so this becomes
nine hundred and stick it here
one hundred and the way we'd represented
here we have ten hundreds we're taking
one of the hundreds we have nine
hundreds left and we're putting that
hundred here so it's ten tens
and I really want you to understand that
this nine is the same thing as 900 why
because it's in the hundreds place this
four is the same thing as four thousand
this is a four thousand because it's in
the thousands place this ten is the same
thing as in the as a hundred because
it's in the tens place it's ten tens and
we're almost done now why don't we take
one of these tens so we have nine left
and put that 10 in the ones place we if
we take one of the tens one hundred
becomes a ninety we put it into the tens
place and now we are ready to subtract
now we are ready to subtract let's see
it becomes 10 minus 4 is 6
90 minus 70 is 2900 - nothing is 900 and
then four thousand minus 2000 is two
thousand and we can do it here 10 minus
four six nine minus seven is 2 9 minus 0
is 9 4 minus 2 is 2 2926 2926 same
number and frankly this is one of the
harder problems you encounter and you
know when if you just learn oh boy you
know I did take away bar all that you
get confused because you try to borrow
from a zero what do I do but when you
think about it in terms of no I'm just
going to regroup I'm just going to take
a thousand from here and put it into the
hundreds place we took a thousand from
here and we wrote the thousand in the
hundreds place and then we took a
hundred from here and put it in the tens
place and then we took ten from here and
we put it in the tens place that's all
we did we just rearranged where the
numbers are what what where what place
value on each of our each of our yeah
numbers are let's do another one
let's do let's do I don't know let me
pick a good one let's do seven well let
me do the problem that they give us five
thousand two hundred minus nine hundred
and forty eight so that equals five
thousand plus two hundred plus zero tens
plus zero ones and then the bottom
number is nine hundred plus 40 plus
eight so once again where are we gonna
need to borrow well I'm pretty much in
the hundreds tens and ones place because
all of these numbers are bigger than all
of these numbers so let's take a
thousand from the five thousand we have
four thousand left and put it into the
hundreds place so we took a thousand so
we have to put it someplace so let's add
it here so a thousand plus two hundred
is equal to twelve hundred and using our
place notation over here the five
thousand becomes a four thousand and
that the two hundred becomes a twelve
hundred and now let's borrow a hundred
from the twelve hundred so we have 1100
we have to put it someplace we'll put it
here so this becomes one hundred or you
could say we borrowed a hundred from the
twelve hundred so we had eleven hundred
left and then we're putting the hundred
in the tens place so it's ten hundred
that's hundred is the same thing as 10
sorry ten tens right there and then
finally let's borrow ten from this
hundred so it becomes 90 and put the ten
there so that becomes ten similarly
let's take ten from here so it's nine
tens and put the ten here now we are
ready to subtract 10 minus eight is two
ninety minus 40 is fifty eleven hundred
minus 900 that's two hundred and then
you have four thousand here 10 minus 8
is 2 9 minus 4 is 5 11 minus nine is 2 4
minus nothing is four 4250
to 4252
hopefully you have a sense of what's
going on now and and frankly these are
if you find these pretty straightforward
you are ready to tackle almost any
subtraction problem especially with four
digits see you in the next video