Video Transcript
Use the number line to find the
value of 20 multiplied by four. Then we’ve got five possible
answers: 24, 60, 80, 100, or 40.
In this question, we’re being asked
to multiply a multiple of 10 by a one-digit number. And we’re told to use the number
line to help us. Let’s take a moment to look at this
number line. We can see that it’s labeled from
zero all the way up to 100. And each interval is worth 10. That’s why each of the numbers
that’s marked is a multiple of 10: zero, 10, 20, and so on.
Now, how can we use this number
line to help us find the answer to 20 times four? Well, we can think of this question
as asking us to find four jumps of 20 on our number line. Now, how many intervals of 10 would
we need to cross to make one jump of 20? How many numbers would we have to
count along our number line? One, two. Because each interval is worth 10,
we move along two numbers for every 20. Now, the reason why we’re saying
this and not just counting along the number line is that we can actually work out
the answer before we start.
If we move along two numbers for
every jump of 20 and we need to find four 20s, then the number of numbers that we’re
going to move along our number line is the same as two times four. We’re going to end up eight numbers
along, which is the same as eight intervals along. And because as we’ve said already
each interval is worth 10, we’re going to arrive at a number that’s worth eight
times 10. We can predict we’re going to end
up at the number 80. Let’s actually use our number line
the way it’s supposed to be used. We’re going to count along in 20s
four times. And let’s see whether we end up at
80.
So we’ll say zero and then 20, 40,
60, 80. We were right! We predicted that we’d need to move
eight numbers along our number line to find the answer. And because our numbers increase by
10 each time, we predicted that the number we’d end on would be worth eight times
10. If two times four equals eight, we
know 20 times four must have a value of 80.
