Video Transcript
Complete: 10 multiplied by what
equals 120.
In this question, we’re multiplying
a number by 10 to get the answer 120. But what is our missing number? Often, when we’re finding a missing
fact in a multiplication like this, we could use division to help, the inverse
operation. In other words, we could divide 120
by 10 to find the answer. But there’s another way to find our
missing number. And that’s to use what we know
about what happens to numbers when they’re multiplied by 10.
We know that when a number is
multiplied by 10, each of its digits becomes worth 10 times as large as it did
before. So just as an example, if we
multiply eight by 10, the digit eight is no longer worth eight ones. It’s now worth 10 times as much,
eight 10s. We started off with a digit eight,
and our answer also contains a digit eight. It’s just shifted one place to the
left. Now, if we look at our calculation,
we know that this is exactly what’s happened to the number we’ve multiplied by
10. Its digits must have shifted one
place to the left. And the answer is 120. This is the number we’ve ended up
with.
The digit two in our answer has a
value of two 10s or 20. But this is after it’s been
multiplied by 10. What was it worth before it was
multiplied by 10? Instead of two 10s, it must have
had a value of two ones. The digit one in our answer has a
value of 100. But this is 10 times greater than
what it was to begin with. So in our starting number, the one
must have had a value of one 10. It looks like the number that we’ve
multiplied by 10 is 12. And as we’ve multiplied it by 10,
each digit’s value has become 10 times as large and has shifted one place to the
left.
We’ve used our knowledge of how
digits change when we multiply a number by 10 to help find the missing factor. 10 times 12 equals 120. Our missing number is 12.
