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.
