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

before we move on I just want to make
sure you have a deep understanding of
how we actually computed 600 whoops
that's not what I wanted to do 600 why
is my pen malfunctioning let's see 698
no that's not the word what am I not
only is my pen malfunctioning my brain
is as well 686 minus 298 and I drew out
the place values and everything last
time but I want to show you that you
don't have to do that but you can still
have the exact same intuition so the way
you should do this type of problem and
in a few videos we're gonna start doing
this with four-digit numbers and that's
why I want to make sure you have you
understand it properly with three-digit
numbers is you the way I did last time I
said oh well you know first I have to
subtract in the ones place and I have an
8 here and 6 here so let me get some
tens from the tens place and then I you
know this became but what I want to show
you is you can actually do all of the
regrouping or all of the borrowing
depending on how you want to call it
although I like to call it taking ahead
of time rather than borrowing cuz you
don't give it back so how do we think
about it
so in order to do any subtraction
problem you just have to make sure that
the number on top is bigger than the
number on the bottom right so for
example in the hundreds place here and
we're not going to negative numbers yet
just yeah well you'll learn that in a
couple of years or if you were
especially advanced maybe even faster
but we see here that in the hundreds
place this number is lower than this
number so that's cool but then when we
go to the tens place what happens here
this 9 which represents 90 that's bigger
than this 8 which represents 80 right so
how can we subtract 90 from 80 and
you'll learn later that that'll give you
a negative number and all of that but we
want to stay in positive numbers so we
have to have a number of larger than 90
here we only have 80 here or we only
have 8 tens so what we can do is we have
600 right here right this six represents
six hundreds so let's take one of them
so we'll take so we'll take one of the
six hundreds and we're left with just
five hundreds but now we have a hundred
to play with and let's put those
hundreds in
tenth place well a hundred is how many
tens 100 is 10 tens right so if we add a
hundred to 80 we get what we get a
hundred and eighty but if we write that
as tens that's eighteen tens so this
becomes 18 tens right let me and and I
really want you to understand I don't
just do this mechanically I really want
you to understand what's going on here
when we took when we took this six and
turn into a five we took one from the
hundreds place one from the hundreds
place represents 100 so when you take
that hundred and represent it in the
tens place it becomes ten tens right 100
is 10 tens so we have eight tens and we
add ten tens to it so then you get 18
tens right that one I mean all I'm
saying is that 100 becomes ten tens
right 10 times 10 is 100 and we added
those 10 tens to the eight tens that we
already had here we got 18 so now the
five is greater than the two 500 is
greater than 218 tens or 180 is greater
than nine tens or ninety but then here
we have six is smaller than 8 so we're
not done yet so what can we do how can
we add more ones here well we could take
a 10 from here we could take a 10 from
here and then and then put it into the
ones place so let's take a 10 away so
we're gonna take a 10 here so we only
have 17 tens left right because this is
the tens place so 110 is equal to right
you can almost use a $10 bill right we
had 18 $10 bills now we took one of
those ten-dollar bills and we have 17
and so that equals 1 and that equals 10
once right a $10 bill is equal to 10 $1
bills and so we took 1 10 from here and
then we can put it into the ones place
so we have 10 more ones so then this
becomes well we had 6 or so you know we
could say 6 dollars and now we're gonna
add 10 to it so we're gonna add 16 you
may have learned you know before that
you're taking a 1 and you could put the
1 to the 8th but why is that really
happening because you took a 1 from here
but that one is a hundred which
represents ten ones yes sorry that one
is 107 represents ten tens so ten tens
plus eight tens was eighteen dance and
then similarly we took a ten from here
and so we have seventeen tens and then
and then we took that one ten turned
into ten ones and added to the six I'm
probably confusing you but hopefully
you're getting the intuition so now
we're in a situation where the five is
greater than the two the seventeen is
greater than the nine and the 16 is
greater than the eight and so we're
ready to subtract and we can do it in
any direction we want to we could start
from the left and go to the right we
could start from the right and go to the
left let's I don't know since it's a
little unconventional start from the
left what's five hundred minus 200
what's 300 right because we're in the
hundreds place what's 17 tens minus nine
ten so that's the same thing is 170
minus 90 alright well that's 80 or eight
tens or you could say 17 minus 9 is 8
and then finally what is 16 minus 8 well
that's just 8 so there you have it that
was a problem we just did and hopefully
that clarified things a little bit so
let's move to the next problem so
problem number five and on the same page
is taking me 20 minutes just to stay on
the same page a man an unnamed man
that's kind of suspicious a man sold 230
balloons balloons at a carnival in the
morning that's kind of that's kind of
unimportant information we don't have to
know that it was at a carnival in the
morning that's just there to confuse you
so I'm just gonna write that a man sold
230 balloons and I don't care where he
sold it or what time you sold it he then
says he sold he sold another 86 balloons
in the evening once again I don't care
what I'll say so this was in the morning
in evening the question they asked us is
how many balloons did he sell at all how
he did he sell in all well if he sold
200 let's say in the morning which I
didn't right now but let's let's say
that this is in a man sold 230 balloons
in the morning so that's that part of
the statement so we could represent it
with these Singapore Math bars that they
like to do so let's say that that's that
has length of 230 and the unit's here
balloons maybe that should be another
unit of length the balloon that's 230
and then it says he's old another 86 in
the evening he sold sold another 86 in
the evening so let's use one of these
Singapore Math bars so he sold another
86 like that so he sold another 86 in
the evening and they want to know how
many did he sell in all so how many did
he sell at all let me pick a new color
how many did he sell at all what's going
to be this number that he sold in the
morning plus the number he sold in the
evening it's gonna be this whole length
so it's essentially going to be the sum
of 230 and 86
so 230 and 86 the total number of
balloons he sold so 230 plus 86 230 plus
86 that 8 should be right below the 3 so
okay let's add the ones place so 0 plus
6 so that's 6 ones all right 0 ones plus
six one two six ones so three tens plus
eight tens right cuz we're in the tenths
place that equals eleven tens all right
but 11 tens we can't write it up we
can't write 11 right here in the tenths
place so what we do is so you know this
three put this 3 plus 8 that's the same
thing as 30 plus 80 right cuz we're in
the tenths place and we know 30 plus 80
you know that equals 11 times equals 110
but we can't write a hundred we can't
write 11 here 11 it's not a digit so
what we could do is we could take a
hundred of this
and turn it and and and turn it into one
so this is you can read it is 100 plus
ten right or you could view it one in
the hundreds place and a ten so what we
can do is let's just put the 110 down
here and then the hundreds place and you
might have learned this is you know
carry the one put it up here right it
kind of you know when I was learning
math in elementary school they would say
oh 3 plus 8 is 11 so put the 1 down
carry the 1 but essentially what you're
doing you're saying 30 plus 80 is 110
and 110 is 100 and 110 so we're putting
the 10 down here because that's the tens
place and then the hundred we're adding
to the hundreds place so that's what
carrying the one really is and I want
you to have that intuition although you
know if you just said carry the 1 it's
real easy 3 plus 8 is 11 carry the 1 and
now we have a 1 in the hundreds place
this is a hundred plus of two hundred
that equals three hundred so three
hundred sixteen that's how many balloons
the unnamed man sold in all after the
carnival I will see you in the next
video