Video Transcript
Write a subtraction sentence that
matches the given model. Three equals five take away
two. Five equals eight take away
four. Three equals eight take away
five. Five take away three equals
eight. Or eight equals three take away
five. Which of the following is another
subtraction equation for the model? Eight take away five equals
four. Eight take away four equals
four. Five equals eight take away
four. Or eight take away three equals
five.
This question is all about the
model that we can see in the picture. So, let’s spend some time looking
at it and seeing what’s going on. To begin with, we’ve got a line of
cubes. Let’s count how many we’ve got
altogether. One, two, three, four, five, six,
seven, eight. So, the whole line of cubes
contains eight cubes, but we can see that it’s then split up. Can you see that it’s been broken
off? So now we’ve got a group of green
cubes and a group of red cubes. Let’s count how much we’ve got in
each part.
We’ve got one, two, three green
cubes and one, two, three, four, five red cubes. You know, we could draw a
part–whole model here to show what’s going on. The whole line of cubes, as we’ve
said already, is eight. But then, when it’s broken into two
parts, the part of green cubes is worth three and the part of the red cubes is worth
five. Five and three are two parts that
go together to make eight.
Now we’re asked to write a
subtraction sentence that matches this model. Now before we do that, what do you
think is being taken away? Do you think the green cubes are
being taken away and we’re left with the red cubes? Or do you think the red cubes are
being taken away and we’re left with the green cubes? It might be hard to see just by
looking at the picture, but there are some things we do know. Firstly, we know that number of
cubes that there are in the whole amount is eight. So, our subtraction is definitely
going to include the number eight, isn’t it?
Can you see any subtractions that
don’t include the number eight? Well, this first one doesn’t, does
it? We’ve got a three, a five, and a
two. Let’s cross off this subtraction;
it can’t be right. Something else we know about our
model is that it contains three green cubes and five red cubes. So, we know our subtraction is
going to include the numbers eight, three, and five. Are there any answers that don’t
include these numbers? Well, if we look at the second one,
we can see that there is a five and an eight, but there isn’t a three. So, again, we know straightaway
this isn’t going to be the correct answer.
So, we’ve got three possible
choices. Let’s have a look at them. The first subtraction says three
equals eight take away five. This is interesting. It starts with the answer, doesn’t
it? Often, we’ll have a subtraction
where we’ll say something take away something equals something. Well, this subtraction is the other
way round. Something equals something take
away something. Let’s read it carefully. Three equals eight take away
five. In other words, three is what’s
left if we start with eight and we take away five. It looks like this matches the
model, doesn’t it? Three green cubes are what’s left
over if we start with eight cubes and we take away the five red cubes.
And if we quickly look at the other
two subtractions, we can see they don’t actually make sense. If we start with five and then take
away three, how can we have eight? That’s a bigger number. And the last subtraction doesn’t
make any sense at all. Eight is what’s left over if we
start with three and then take away five. I think we can cross this one
through, don’t you? So, the subtraction that matches
our model is three equals eight take away five.
In the second part of the question,
we’re asked which of the following is another subtraction equation for the
model. Let’s do exactly what we did last
time. Remember, we’re looking for a
subtraction that includes the numbers eight, three, and five because this is the
whole and the two parts that we’re talking about. The first subtraction contains an
eight and a five, but not a three. The second one we don’t have either
of the two parts that we’re looking for. We’ve just got two fours. In the next subtraction, we’ve got
a five and an eight again, but not a three. And so, the only subtraction that
matches our model, which shows that three and five go together to make eight, is the
last one.
This time, it’s as if we’ve taken
away the green cubes, isn’t it? Eight cubes subtract three green
cubes leaves us with five red cubes. In the model, it wasn’t very clear
whether we were taking away the red or the green cubes. But one thing we did know is that
we had eight cubes to start with, and we split it into three and five. And that’s how we know that the
subtraction sentence that matches the model is three equals eight take away
five. But we can also say eight take away
three equals five.
