# Question Video: Multiplying One-Digit Numbers by Multiples of 10 Mathematics • 4th Grade

What is the result of 7 × 3? What is the result of 7 × 30? What is the result of 70 × 30?

03:13

### Video Transcript

What is the result of seven times three? What is the result of seven times 30? What is the result of 70 times 30?

We’ve got three multiplication questions to answer here. Do you notice anything similar about them? The digits seven and three keep cropping up again and again, don’t they? The reason for this is because this question is all about using a fact like seven times three to help us find the answer to related facts. And so when finally we arrive at multiplying these two multiples of 10 together, we’ll see that 70 times 30 has got a lot to do with seven times three.

So let’s start off with the most basic of these multiplication facts. What is the result of seven times three? Three, six, nine, 12, 15, 18, 21. Seven threes are 21. Now if we look at the second question we’ve got here, we can see that one of our numbers has changed. What is the result of seven times 30. Now if we write out this calculation, we can see how it’s linked with the first one. Instead of finding the answer to seven times three, we need to find the answer to seven times 10 lots of three, seven times 30. And because one of our factors has become 10 times larger, we’d expect the answer to become 10 times larger, too. So instead of 21, we get the answer 210. When we multiply a number by 10, the digits shift one place to the left. And that’s why 21 becomes 210.

Finally then, we need to find the result of 70 times 30. Now at the start, we probably looked at this question and thought we’re multiplying two multiples of 10 here. This is really tricky. But because we’re working through this step by step, it doesn’t seem so difficult anymore. We know what seven times three is, and we know what seven times 30 is. So it’s not such a big step, really, to move to 70 times 30. All we’ve done really is make the number seven 10 times larger. And again, if we make one of the factors 10 times greater, the answer’s going to be 10 times greater, too. So instead of 210, we’re going to need to shift those digits one more place to the left. The answer’s 2,100.

This question has taken us through some multiplications step by step. And it’s taught us that if we want to multiply two multiples of 10 together, we could use a fact we already know to help us. Seven times three equals 21. And if we know that, we know that seven times 30 equals 210. And if we know that, then we also know that 70 times 30 equals 2,100.