Singapore Math: Grade 3a; Unit 1 (part 9) Video Lecture | Mathematics Lesson for Grade 3 (Singapore) - Personal Learning

let's do a problem similar to what we
just finished doing in the previous
video but instead of adding or saying
you know what it is a hundred more than
a given number or a thousand more let's
say what happens when we take away what
happens when we take a number that's a
thousand less or a hundred less in a
given number so let's pick our number to
be five thousand seven show using the
right tool 5732 so first of all let's
write down five thousand five hundred
thirty two in terms of our in terms of
our poker chips or in terms of our cans
of marbles so five thousand let's see if
I can draw five thousand it's green so
it's one two three four five right and
each of those are a thousand each of
those are a thousand have seven hundred
that's gonna take me a little time to
draw seven hundred one two three four
five six seven hundred those last two
overlap 100 100 100 hundred hundred
hundred hundred I have three tens it's
one two three tens
ten ten ten and I have two ones that's
easy
two ones so one to two ones
so let's say what happens when I what if
I were to ask you what is I don't know
1000 less than five thousand seven
hundred thirty-two well this is 5732 so
if I take if I say what is one thousand
less what am i doing well I'm
essentially going to take away a bucket
of a thousand or or a can of a thousand
marbles or I'm going to take away a a
one thousand dollar poker chip so let's
look at our thousands well we have one
two three four five thousands I'm gonna
take one of them away so let me take and
let me take this one let me I'll just
cross it out
so I'm going essentially at what's one
thousand less let me take away one of
these so I'm going to take this one away
so what's the resulting number well how
many thousands do I have left if I took
this one away I have one two three four
four thousand how many hundreds I still
have seven hundredths right
one two three four five six seven didn't
change that three tens thirty-two ones
two 4732 and if we if we didn't do it
using our poker chips or using our cans
of marbles you could just say well if
you took a thousand away from 5732 you
essentially just take a you take one
from the thousands place this is
equivalent of taking away one thousand
so we took away one from the five and we
got four here so 4732
fair enough let me erase this I did not
want to erase with the yellow I'll have
to draw that that thousand chip back
erase it first let me draw that back
so the original number is 5,000 so
that's what I wanted that to be back
there so what happens if instead of
saying what is a thousand less than five
thousand seven hundred thirty-two what
happens if I ask you what is I don't
know what is one less what is one less
than five thousand seven hundred
thirty-two well just like when we did a
thousand less we took away a thousand
dollar poker chip if we want one less
we'll take away a one dollar poker chip
so essentially we could take away one
chip right I took that away so what do
we have left we have five thousand and
I'll just do it in one color well I
never actually did this with five
thousand seven hundred still seven
hundred thirty three tens is thirty and
we took one away here so we only have
one left
I'm just counting them so we have five
thousand seven hundred thirty one which
makes sense you take away one from 5732
you're taking one away from the ones
place so the two goes down by one to
become a one right here so let me add
back that that $1 that I took you out to
do another problem so what happens if I
were let me erase some of this so what
happens if I were to ask you what is
what is one hundred less than five
thousand seven hundred and thirty-two
well I'm going to take away a hundred
chip right so let me just I'll just
erase it I've always been erasing the
bottom one but I could erase any of them
so I'm going to take one of these seven
hundred s away I erased that one it
disappeared doesn't exist anymore so now
what do I have left I have five thousand
five thousand how many hundred to have
one two three four five six 600 that
makes sense I had seven I took away I
have six now I had seven I took one of
the hundreds away I have six
now three tens thirty-two ones let's do
one left one more before I do that I
have to add that hundred back that I
just took away add the hundred there we
go let me erase this erase that let's
say what is ten less what is 10 less
than 5732 well that means we take away a
$10 chip away from from our collection
of chips so what is 10 less I'm just
erased one of the tens that's gone so
now we have five thousand seven hundred
we have two tens two tens and two ones
so five thousand seven hundred and 222
is what we get when we take 10 away from
5732 which makes sense because we took
ten away
so we decreased the tenths place by one
to get five thousand seven hundred and
twenty two plus is a couple of exercises
now that I think you have a familiar
familiarity with what happens when we
add ten to a number or take away ten
from a number or add a hundred or a
thousand or one let's see if we can
count by a hundred or a thousand or by
one or different orders so what I want
you to do is count in steps of 10 so
count in steps of 10 from 1678 to 1728
let's see if we can do that so the first
it's 1678 so we're counting the steps of
tents we're adding 10 every time so when
you add 10 you add you increase the tens
place by one so one thousand six hundred
seventy eight when you add 10 becomes
1688 where we just added 10 let's add 10
more we get 1698
and now once again if we add if we just
incremented this digit by one we get a
ten here but you can't have two hen you
can't have ten tens but you wouldn't
write it that way what happens when you
have ten tens that means you have an
extra hundred right ten tens equal 100
so now instead of incrementing this and
getting a ten somehow in the tens place
we say oh well we're gonna have ten tens
so let's increment the hundreds place so
we get one thousand seven hundred and we
used all of these plus one more to get
an extra hundred so now we have zero
tens eight so it's one thousand seven
hundred and eight and now from here it's
actually pretty straightforward we keep
incrementing the tens place again 1718
1728 one thousand or that's where we
wanted to go we're done let's do an
another one I'll switch colors let's
let's step in a hundred counts count in
steps of a hundred from 1678 to 2178
let's see what we can do so we have 1678
we were incrementing by a hundred so we
add one to the hundreds digit every time
so we know they have 1778 1878 1978 and
now something interesting happens we are
going to add another hundred so we would
have ten hundred but ten hundred once
again you can't have a ten in the
hundreds place you can't put a two-digit
number there and ten hundred is also
equal a thousand so this is going to
actually this instead of having nine
hundred we say well we have one more
thousand so we take those nine and r
adding another hundred we get another
thousand so we increment the thousands
place we get two thousand now
Enzo and and we use all of these up plus
the extra hundred to get that extra
thousand so now we were going back to
zero hundreds 78 and then we keep adding
hundreds again 2178 let's do one more I
want to count in steps of a thousand
from six 1678 to eight thousand six
hundred and seventy eight this is pretty
straightforward we start 1678 what's a
thousand plus that well when you add a
thousand you're essentially adding one
to the thousands place so you get two
thousand six hundred seventy eight three
thousand six hundred and seventy-eight
four thousand six hundred seventy-eight
5678 I think you get the point six
thousand six hundred seventy eight 7778
and then we get eight thousand seven
hundred hi well actually I messed up
even the best of us could make careless
mistakes I actually incremented the
hundreds place here six thousand six
hundred seventy eight were only
increasing but the thousands place we
get seven thousand six hundred and
seventy eight and then we get eight
thousand six hundred and seventy eight
you can't do anything too quickly
alright now this unit we just have one
more set of exercise and I think it's
good you know so I'm I just want to do
all of them so I'm gonna give you a set
of numbers and I want you to complete
the blank so let's say we went from
three thousand ninety eight to three
thousand nine and half a blank here then
I got three thousand one hundred and one
and then three thousand hundred and two
but commas here so you know these are
different numbers so what do we do we go
from 3098 to 3099 what did we do we
increased by one right and I'm going
from three thousand 101 to three
thousand 102 we also increased by one
so essentially this looks like we're
just increasing by one every time so
what's one more than 3000 99 let's
remember we have when you add 1 to 99
you're essentially getting a hundred
right so you could say three thousand
one hundred and it makes sense then then
when you add one to that you get 101 102
and this was the hard one to figure out
right because when you increment the
ones place by one you get attend there
and you say oh no now I have ten ones so
I have an extra ten and so you you wanna
so you instead of writing this two
thousand nine ten you can't write that
so you you exchanged those tens those
ten ones for an extra ten so then you
might say three thousand ten zero but
you can't have that either so exchange
those ten tens for a hundred and you get
three thousand one hundred and nothing
else and that's what we got there let's
do that a couple more times
so let's say we have no 3098 3108 3118
let's say blank and then 3138 put some
commas in so what happened when we went
from three thousand ninety eight to
three thousand one hundred and eight
what did we do what's the difference
between three thousand ninety eight and
3108 the ones place is the same but if
we look at the tens place what happens
when you increment this by one when you
increase the tens place by one you can't
put a ten there right so when you have
you have nine tens here this is a tens
place you increment when you have ten
tens well it's ten tens it's a hundred
so you could you exchange all of those
add the extra one and you get an extra
one there so all we did here is we
increased by ten and you see that here
again 1108 1118 we increased by 10
because the tens place increased by one
so let's increase by 10 again
we just increment the tenths place 3128
let's do one more of these and they're a
couple more in the workbook but I think
you you get the idea start with the same
number three thousand nine the 3098 it's
a blank and I really encourage you to
pause these before I tell you how to do
them see if you can do it themselves
five thousand ninety eight six thousand
ninety eight seven thousand ninety eight
well this might be obvious what's the
next one is because we you just have one
number so you know what the pattern is
but you go five thousand six five
thousand ninety eight six thousand
ninety eight seven thousand ninety eight
so it sounds like after every one we're
just increasing the thousands by one
right we're adding a thousand so this is
3098 let's add a thousand four thousand
ninety eight five thousand ninety eight
that's it let's do actually we've been
doing actually I think I want to do one
or two more just to make sure we get all
the concepts in so let's say I have
eight thousand nine hundred and three
and then I have a blank number here and
then I have eight thousand eight hundred
eighty three eight thousand eight
hundred seventy three and then eight
thousand eight hundred sixty-three so
there's a couple of ways we see it's
hard to figure out a pattern when you
only have one number here but we can
look at these what happens we're getting
eight thousand eight hundred eighty
three eight thousand eight hundred
seventy-three so we're we decreased the
tens place by one and we have eight
thousand eight hundred sixty-three we
decrease the tens place by one so we're
decreasing by ten every time minus 10 so
this number right here must be ten more
than this number
so let's ten more than this number well
we increase the tens place by one and we
get eight thousand eight hundred and
ninety three and this is interesting
because
ten from here how do we do that how do
we subtract 10 from here from eight
thousand nine hundred and three well we
have nothing in the tens place right
there so what we do is we take one of
these hundreds so we only have 800 left
we take one of them we exchange them for
ten tens and then we take one of those
tens away so we say this is the same
thing as eight thousand eight hundred
and then ten tens and a three right if
we think of the places is the tens place
and now we decrease it by one and we get
this eight thousand eight hundred and
ninety-three I think that gives you a
good intuition of this this video is run
very long so I will see you actually in
the next unit see you soon