Video Transcript
Subtraction Equations
In this video, we’re going to learn
how to read, write, and model subtraction equations with numbers up to 10.
Here’s a group of balloons. It contains one, two, three, four,
five, six balloons. Now, let’s imagine a hedgehog comes
along. But the prickles on a hedgehog and
our six balloons don’t mix very well. What if the hedgehog pops two of
the balloons? One, two. Now, how many balloons are
left? We know that the number of balloons
that are left will be the same as six take away two. They’re going to be one, two,
three, four balloons left.
We’ve subtracted or taken away two
from six to give the answer four. Six take away two equals four. But, you know six take way two
always leaves four. If there are six birds and two of
them fly away, we’re left with four birds. It doesn’t matter if we model a
subtraction using six counters or six cubes; when we subtract two from six, the
answer is always four.
Now, what if there was a way for us
to show that six take away two equals four without us having to draw balloons or use
counters. Wouldn’t that be useful? Well, we can do this. And we do this by writing what’s
called an equation. And because we’re thinking about
taking away and subtracting, this is going to be a subtraction equation. Now, the word equation might sound
tricky, but all it means is a way to show a number fact using numbers and
symbols. In other words, we’re going to show
that six take away two equals four without drawing any pictures, without writing any
words, just using numbers and symbols.
The first thing that we can do is
to show that number that we start with. This is the number that we had
before we took anything away. And in this number fact, it’s the
number six. So, we’ll write the digit six. Now, we need a symbol to show that
we need to take away two. And we can use the subtraction
symbol. The subtraction symbol is a
horizontal line that looks like this. This symbol means take away or
subtract. And, of course, we know we’re
taking away two, so we can write the digit two after this symbol.
Next, we need a symbol that
represents “is equal to” or “is the same as.” Six take away two is the same as
four. So, we can use the equal sign. We draw two lines. And the final part of our equation
is the number that we have left, which is four. Six take away two is equal to
four. You know, we can show any
subtraction fact we want using an equation like this. Let’s practice answering some
questions where we have to think about subtraction equations.
Which picture shows five take away
four equals one?
In this question, we’re given a
subtraction equation or a number sentence. An equation or a number sentence is
a way of showing a number fact using numbers and symbols. In this particular number sentence,
we can see the number five then this symbol that looks like a short line. This is the symbol for taking away
or subtracting. Then, we have the number four. This is the number that we need to
take away.
Then, we can see the equal
symbol. And after that comes the
answer. This is the amount that we have
left. We can read this number sentence as
five take away four equals one. So, which picture shows five take
away four equals one? And as we’ve said already, the
number five represents the number that we’re starting with. Let’s go through each of our
pictures to see which one contains five objects to begin with.
Our first picture shows one, two,
three doughnuts. This picture doesn’t start with
five, does it? Let’s look at the group of
dogs. We have one, two, three, four, five
dogs. This picture could show five take
away four equals one. In our second set of doughnuts,
there are one, two, three, four, five. This could be the correct one. Then, we have a group of one, two,
three, four sweets. This is too small. And our final picture shows one,
two, three, four, five sweets. So, the picture that we’re looking
for is either the row of dogs, the longer row of doughnuts, or the longer row of
sweets because they all contain five objects to start with.
The second number in our
subtraction equation is the number four. And this shows us the number that
we need to take away. In these pictures, we can see the
objects that have been taken away because they’ve been crossed through. How many dogs have been taken
away? One, two. The picture of the dogs shows the
subtraction five take away two. What about the long row of
doughnuts? We’ve taken away one, two, three,
four. This shows the subtraction five
take away four. This is the subtraction we’re
looking for. Looks like this is going to be our
answer. Let’s just check the row of
sweets.
Well, if we look, we can see that
only one sweet has been crossed out. This shows the subtraction five
take away one. The last number in our number
sentence is one. Five take away four equals one. And we can see in our row of
doughnuts that this is true. Five doughnuts take away four
doughnuts leaves us with one doughnut. The picture that shows five take
away four equals one is the row of doughnuts that shows five doughnuts with four of
them crossed out and one left over.
Yesterday, Alice had four
sweets. She ate two. How many are left? Find the matching number
sentence. Two plus one equals three, four
take away three equals one, two plus two equals four, four take away two equals two,
or four take away one equals three.
Our problem starts by telling us
that yesterday Alice had four sweets. But she doesn’t have four
anymore. She ate two of them. And we can see this in the picture,
can’t we? There’s a row of four sweets and
two of them have been crossed out. These are the two that Alice has
eaten. Our first question asks us, how
many are left? Let’s count the sweets that haven’t
been eaten. There are one, two sweets left. So, Alice started off with four
sweets, she ate two of them, and she’s got two left.
In the second part of our problem,
we’re asked to find a matching number sentence. A number sentence or an equation is
a way to show a number fact using numbers and symbols. The first thing we can do here is
to think about what sort of number fact are we looking for. We know that Alice started off with
four sweets and then she ate two of them. Another way of thinking of this is
that she’s taken away two of them.
The answer’s going to be less than
the number she started with because she’s eaten them. So, we’re looking for a number
sentence that shows a subtraction fact or a take away fact. And the symbol that represents
subtract or take away is a little line like this. We can see this symbol in three of
our possible number sentences. The others show an addition
symbol. But this isn’t an addition
fact. Alice isn’t adding her sweets, so
let’s cross through the addition number sentences. We’re looking for a subtraction
number sentence.
Alice starts off with four sweets,
she eats two sweets, and she has two sweets left. Which number sentence shows four
take away two? It’s this one here. We started off by finding that
Alice has two sweets left. And the matching number sentence is
four take away two equals two.
There were three cats on a
roof. Two of them jumped down. There was one cat left on the
roof. Fill in the blank. Three take away what equals
one?
In this question, we’re given a
number story. But as well as that, we’re given a
number sentence or equation with a missing number in it. And the two things go together. To start with, let’s think about
the story. Our story begins with three cats on
a roof. Here are three counters to
represent our three cats. In the next part of our story,
we’re told that two of the cats jumped down. Suppose we could represent this
with our counters by taking away two of our counters cause the cats aren’t on the
roof anymore. One, two.
The final part of our story tells
us there was one cat left on the roof. This is shown in our model because
we have one counter left. We had three, we took away two, and
now we’ve got one.
Now, let’s look at the number
sentence or equation. We’re told to fill in the
blank. If we look carefully at this
subtraction equation, we can see that it has a missing number in it. Three take away what equals
one? In our story, what did we take away
from three that left us with one? Well, the number three represented
the number of cats that were on the roof or the number of counters that we had in a
row to start with. This means that our missing number
is the number that we took away from three. This is the number of cats that
jumped off the roof.
We know that two cats jumped
down. That’s why we subtracted or took
away two counters. And the final number in our number
sentence is the number that we have left. If we subtract two from three,
we’re left with one. The number sentence that matches
our story about the cats is three take away two equals one. Our missing number is two.
So, what have we learned in this
video? Well, firstly, we’ve learned that a
subtraction equation or number sentence is a way to show a subtraction fact using
numbers and also symbols. And so, we’ve learned how to read,
write, and model subtraction equations with numbers up to 10.
