Video Transcript
Use place value to subtract
numbers. Pick the correct way to break
apart 47 into tens and ones. Subtract five from 47 by taking
away five ones.
The main calculation that we
need to do in this question comes right at the end. We need to subtract five from
47. But before we get to this step,
there’s one or two things we need to do first. The first sentence tells us
that we need to use place value to subtract numbers. Do you remember what these
words “place value” mean? They’re all about how the
digits in a number can have different values depending on where you write them,
depending on their place in a number. And we can see, just by quickly
looking at this question, we’re thinking about two-digit numbers.
In the first part of the
question, we’re asked to start thinking about a two-digit number. We need to pick the correct way
to break apart 47 into tens and ones. We know from later on that 47
is one of the numbers in our subtraction. And it looks like it’s going to
be helpful to us to split up this number into its tens and ones. The diagram shows that two
different place value grids. And each one shows a different
number of tens and ones. But which one of the two is
correct, seven 10s and four ones or four 10s and seven ones?
Hopefully, you can hear the
similarities as we read those out. They both contain seven of
something and four of something else. In other words, they both
represent numbers that contain a seven digit and a four digit. But which way around?
To answer this part of the
question, we need to understand something about the digits in 47 and their place
value. And to help us, we could write
those digits into a place value grid. Our grid shows two spaces, one
for the tens and one for the ones. And we can write our digits in
order. The digit that comes first is a
four, and that’s going to go in the tens place. So 47 has four 10s. We know this because four 10s
are worth 40. And there’s only one place for
us to write our digit seven, and that’s in the ones place. This has a value of seven
ones.
Now, we can use this to help us
find the correct answer. We’re looking for the place
value grid that shows four 10s and seven ones. That’s this one here. The other model shows seven 10s
and four ones. It’s the number 74. But we know that the number our
subtraction begins with can be modeled using four 10s and seven ones.
In the final part of the
question then, we’re asked to subtract five from 47 by taking away five
ones. Can you see how the subtraction
has been written in a place value table for us? Now, the calculation could’ve
been written like this. But by writing the numbers in a
place value table like this, we’re really doing what we’ve done in the first
part of the question. We’re splitting them up into
their tens and their ones. We’re using place value to help
us.
Now, we’re told we need to find
the answer by taking away five ones. And this is because the number
that we need to subtract, five, is worth five ones. If you look in our place value
table, can you see the digit five in the ones place? These are the five ones we need
to take away. So why does this question ask
us to take away a number of ones? Why doesn’t it say a number of
tens and some ones?
Well, that’s because we’re only
taking away a one-digit number. The number five doesn’t have
any tens in it. We just need to subtract a
number of ones. And in the number 47, we
already have seven ones. So we know we’ve got enough
ones to take away our five ones without doing anything else. Let’s cross out five of our
ones blocks and see what we have left. So we’ve got seven to start
with, and then six, five, four, three, two. Let’s write what we have left
in our place value grid. We had seven ones, we’ve taken
away five ones, and now we’re left with two ones. And as we’ve said already, we
don’t need to do anything with the tens.
We started off with four
10s. We don’t need to take any
tens. And four take away zero leaves
us with the same number. We still have four 10s. Our answer contains four 10s
and two ones. It’s the number 42.
In this question, we needed to
subtract five from 47. The first thing we were asked
to do was to break 47 apart into its tens and ones. And we know 47 has four 10s and
seven ones. And by modeling it using place
value blocks, we could then take away five ones blocks. And that’s how we know 47 take
away five equals 42.
