Singapore Math: Grade 3a Unit 2 (part 8) Video Lecture | Mathematics Lesson for Grade 3 (Singapore) - Personal Learning

let's do some subtraction and actually
in these videos I'm going to switch
between the Singapore Math way of
visualizing things and the Salle way of
visualizing things and actually they're
they're the exact same way I find the
sound way to be more intuitive because I
am Sal anyway let's do some problems
let's do 3246 mm-hmm - 1634 and let's do
it with the Singapore Math visualization
of what's going on so this is the
thousands this is the hundreds this is
the tens and this is the ones so let's
draw the top number three thousand plus
three thousand one thousand two thousand
and three thousand two hundred one
hundred two hundred forty so that's four
tens 1/10 2/10 3/10 for tens and then
six one two three four five six and
these are all these are all ones and
we're going to subtract 1634 from it so
whenever we do subtraction and I think
in a couple of videos we sample this a
little bit with three digits we just
have to make sure that the number below
the number above is smaller so that we
can just do subtraction each of the
places but if we go you know you could
go right to left or left right you could
see that well one thousand is less than
three thousand so that's cool six
thousand is not less than two thousand
re six hundred is not less than two
hundred so that's not cool so we're
gonna have to do something here but then
we have thirty is less than 40 that's
okay and four is less than six so
somehow we have to be able to get a
larger number appear in the hundreds
place and the way we do that the way I
you know what I learned it in the
early 80s they called it borrowing and
even that wasn't a real term because
you're not borrowing from a number
another number place you're taking it
you're never giving the number back but
anyway I think now and it's called
regrouping but it's all the same thing
so what do you do for exhibit we were in
the hundreds place right we have two
hundreds and we have to take out six
hundreds from this right so how do we do
that well what we do is we can take a
thousand and we can turn it into ten
hundreds so let's take this thousand now
let's turn it into ten hundreds one two
three four five six seven eight nine ten
each of those are 100 100 100 100
hundred hundred hundred hundred hundred
knots ten hundred it's right so we got
rid of this thousand so how do we draw
it here well we got rid of one thousand
so we had three but we cashed in one of
them so now we only have two left all
right one two and then how many hundreds
do we have we have these ten plus two we
have 1,200 1,200 and now every number on
the bottom is less than the number on
the top and we can subtract so 6 minus 4
so 6 minus 1 2 3 4 is 2 4 minus 3 so we
have 4 minus 1 2 3 is 1 12 minus 6 1 2 3
4 5 6 then we have 6 left and you know
the 12 minus 6 is 6 and then 2 minus 1
this one's not there anymore so we have
to take out one you just have one left
1612 so let's do the other problems that
they have here and I'll skip around a
little bit because I think the important
thing is that you get the big picture
and then after get the big picture the
rest of the problems are really just a
little bit of practice so let's do 5,400
so I'm going to do five thousand three
hundred and forty minus six and now I'm
going to do it Sal's way and you'll see
it's really the same thing as a
Singapore math way it's just a slightly
lift four different way of thinking
about it so how can we write five
thousand three hundred forty what's a
different way of writing it well five
thousand three hundred forty is the same
thing as five thousand plus three
hundred plus 40 plus zero ones right
5340 and we're going to subtract that
how do we write six well six is just six
ones right and there's nothing else in
any of the other places so how do we do
that well in the ones place we have a
zero and we want to take a six away from
it you can't do that and you will in the
future when we do negative numbers but
for what we do right now you can't take
a six from a zero so what do you do well
let's take ten away from the for the
tens place so let's take ten from here
so this becomes 30 and let's put that 10
here and how do we do it here how does
that look well this becomes a three and
then this becomes a ten and now we can
subtract what's ten minus six
what's ten minus six it's 4 + 30 + 300 +
5000 or we do it on this side ten minus
six is four three - nothing is three
three minus nothing is three five -
nothing is five and there you have it
five thousand three hundred thirty-four
is equal to five thousand plus three
hundred plus thirty plus four let's do
another one actually all of these are
kind of of the same nature but yeah well
doesn't hurt to go through them they do
it where you have to borrow in the tens
place and then you have to borrow in the
hundreds place but they're all kind of
the same notion so let's do seven
thousand minus three hundred well if we
write it out like this
seven thousand is really just seven
thousand plus zero hundredths plus zero
tens plus zero ones three hundred is
three
plus zero tens plus zero ones and so we
want to subtract this 300 from this zero
hundreds how do we do that well let's
take one of the thousands from the 7000
so we're gonna take a thousand away so
we only have six thousand left and we're
going to put that thousand here so if we
do that well how does this look on this
side well that becomes a six and this
becomes a ten right ten hundred is a
thousand this ten hundred is the same
thing as this thousand this 6,000 is the
same thing as that six thousand and now
we can subtract 0 - 0 0 0 minus 0 is 0
10 minus 3 is 7 6 - nothing is 6 now if
we did the same thing here is 0 minus 0
0 0 minus 0 thousand minus 300 is 700
and then 6 thousand - nothing is 6,000
so six thousand seven hundred is equal
to six thousand plus 700 plus zero tens
plus zero let's see if we can do a
harder one now this one looks
interesting
problem number six on page 30 problem
number six and I will do it in blue so
two thousand four hundred and
thirty-five - 1268 let's think about
this let's write it and what I call
expanded form but that's 2,000 plus 400
plus 30 plus 5 right 2435 these are
equivalent and then the bottom number is
1,000 plus 200 plus 60 plus 8 and you
see why this is really the same thing as
that Singapore Math where I draw up the
tables and I draw the disks and actually
I'm open to your feedback on which one
you prefer which one you find more
intuitive this one obviously takes less
time and it's something that you're
Michael you know you could probably do
on the side of your paper if you really
want to get into
what's happening so let's try to
subtract so remember the number on the
bottom if you want to subtract has to be
less than the number on the top or the
number right above it 1 is less than 2 2
is less than 4 6 is not less than 3 and
8 is not less than 5 so neither of these
are cool so what can we do these worked
out but we can't subtract 60 from 30 so
what can we do well let's take one of
the let's take a hundred from 400 so
we're going to make that into 300 and
add that hundred here so 130 all we did
is we shifted 100 from the 400 to the 30
that's all we did okay but now we still
have this problem here where the 8 is is
greater than the 5 so let's take a 10
from this 130 and it becomes 120 I'm
running out of space that's 120 and I
took the 10 I'll shift it here so this
becomes 15 so I know you can't read so
this is 300 this is 120 this is 15 and
now we're ready to subtract actually
before we subtract let's see how it'll
look
you know if you weren't doing an
expanded form we took a hundred from the
4 from the 400 and we added it to the 30
so this became a 3 and then the 30
became 130 right 13 in the tens place is
130 and then we said well we need this 5
isn't large enough so let's take 10 from
the tens place so instead of having 13
tens we're gonna have 12 tens and add
add that to the 5 so the 5 becomes a 15
that's exactly what we did at this end
right it's just the only difference is
we wrote the zeros so that you see that
you know 12 tens is 123 hundreds is 315
is course of 315 now we can subtract 15
minus 8 is 7 12 minus 6 is 6 3 minus 2
is 1 2 minus 1 is 1 or 15 minus 8 is 7
120 minus 60 is 60 300 minus 200 is 100
mm minus 1,000 is 1,000 and just so you
remember this one
represents this 1000 this one represents
100 and how do I know that because of
its place its place value this six
represents this 60 and then of course
the seven is represents that seven or
they represent to each other let's do
another one
and now I'll actually Oh actually I'm
already on the 11th actually let me just
keep going cuz I think now that I'm a
YouTube partner they let me do videos
for as long as I want which could be
dangerous I might keep I might keep
going forever I might never stop because
this is just so much fun so I'll do
problem number 10 on page 32 and problem
number 10 we have six thousand six
thousand minus two hundred and
fifty-seven I'm gonna do it the
Singapore math way so let's say this is
the thousands place just right th for
thousands this is the hundreds place
this is the tens tens this is the ones
the ones okay so when we subtract we
want to see if the number above is
greater than the number below well 0 is
not greater than two zeros not greater
than five 0 is not greater than seven so
we have an issue so how do we do this
well first let's just represent the
6,000 over here so 6,000 is six
thousands literally 1,000 2,000 3,000
I'm running making these two big 4,000
5,000 and 6,000 and then we have no
hundreds tens or ones right but we need
hundreds of tens or one so we can
subtract so what can we do well let's
take one of these thousands and convert
them to ten hundreds so I'm going to
take this thousand and convert it to ten
hundreds right ten hundred so I've one
two three four five six seven eight
910 all of those are 100 I think you
know those are 100 I don't have to write
a hundred in each of them so and how
does that correspond here well I took
1,000 so I only have 5000 left and I
converted them to ten hundreds so now I
have a 10 in the hundreds place ten
hundreds well
the hundreds place looks good now but
now the tenths place I have zeros less
than five so what can I do let me take
one of these ten hundreds these newly
cashed out ten hundreds and tournament
two ten tens so if I take this one let's
take that one and I will convert it to
ten tens right this is one hundred that
I just took so that's ten ten so it's
one two three four five six seven eight
nine ten and each of these are ten so
what did I do
I took one of my ten hundreds so now I
have nine and I have ten tens
well that might seem like we're done now
but we still have a problem in the ones
place seven is greater than zero which
is not acceptable so let's do the same
thing let's take one of these tens this
is a ten right each of these are ten
let's take one of them away and convert
into ten ones one two three four five
six seven eight nine ten and so we took
one of these ten tens so we have nine
left
and we have ten ones now we are ready to
subtract ten minus seven is three or ten
minus seven one two three four five six
seven we have three left nine minus five
this one was already gone so one two
three four five is 4 9 minus two is
seven right we take out one two we have
7 left and then five - nothing so 5743
let's do well actually I'm already at
running at 15 minutes so I will do the
next problem in the next video see you
soon