### Video Transcript

Which arrow is pointing at the
number 670?

We’re given five different number
lines here. And can you see? On each one, there’s a blue arrow
pointing out a position. But only one of the blue arrows is
pointing where the number 670 belongs. Let’s go through each number line
to try to find out which one it is.

Now, if we look at our first number
line, we can see that not all the marks on the number line have got a number by
them. We start with the number 600. Then we make one, two, three jumps
before we get to the number 630. Now, we know 630 is 30 more than
600, and 30 is the same as three 10s. So each jump that we’ve made must
be worth 10. 600, 610, 620, 630. The numbers fit. And we’ve worked out that the scale
of the number line is to count in 10s.

Let’s count on in 10s from 660 to
find out what number the blue arrow is pointing to. 660, 670. This is the number we’re looking
for, so we can see straightaway that the arrow isn’t pointing to it. And then 10 more than 670 is
680. The first arrow doesn’t show 670;
it shows 680.

Let’s try looking at our second
number line. This number line is a little bit
similar to the first one in that the first number we can see labeled is 600 and the
last number is 690. But we can see that there are a lot
more notches on this number line, aren’t there? The scale must be different.

Let’s try and work out what the
scale is. If we start at 600, we make one,
two, three jumps before we get to 615. Now, on the first number line, we
made three jumps and we got to 630. That was by adding 10 each
time. But this time, we’ve only got to
615. So it seems like we’re adding less
than 10. Let’s try counting in fives. 600, 605, 610, 615. It works! And so we know that, on this number
line, each interval or mark is worth another five.

Now that we’ve worked out the scale
of the number line, we can work out the value of the arrow. Is it pointing to the number
670? Let’s start with 660, which is the
notch before it, and we’ll count in fives. 660, 665. Oh dear, we can see that the number
the other side of the arrow is the one we’re looking for, 670. On this number line, the arrow is
pointing to 665, not 670.

This next number line is a really
quick one to understand. 100, 200, 300. We can see that the scale is to
count in 100s. And the blue arrow on this number
line is pointing halfway between the number 500 and 700. We know that this arrow is pointing
to the value 600. We’ve only got two more number
lines to look at. Which arrow is pointing to the
number 670?

Now, can you see a similarity
between this number line and the first one we looked at? Each of the intervals is worth
10. On the first number line, not all
the numbers were labeled, were they? But on this one, they are, all
except the one with the blue arrow. 600, 610, 620, 630. Just like the first number line
then, we’re counting in 10s. And we can see that the blue arrow
is the number that comes after 660. And 10 more than 660 is 670. It looks like this is the answer
we’re looking for.

Let’s just check our final number
line. Well, if we look at where the blue
arrow is pointing on this number line, we can actually use some number sense to
realize it’s not pointing to the number 670 at all. It’s pointing to a number that
comes after 675. And because we know 675 is more
than 670, we know that the blue arrow must be pointing to a number more than
675. We don’t even need to know the
scale on this one. We can see straightaway it’s not
going to be 670.

The correct number line is the one
that shows a scale that goes up in 10s each time. And it shows the arrow pointing to
the number that comes after 660. We know this because the number
that is 10 more than 660 is 670.