Video: Multiplying Two-Digit Numbers by One-Digit Numbers Using a Number Line

Use the number line to find the value of 20 × 4.

02:33

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.

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