Video Transcript
What is the missing number? 34 times what equals 68.
In this question, we can see a
multiplication calculation. Did you know often this
multiplication might be written like this? 34 multiplied by something equals
68. We need to find the missing
number. Now these are two ways of writing
exactly the same multiplication. But can you see how it’s been
written in the question? The numbers have been written on
top of each other, vertically. They’re all in columns, and we call
this way of working the column method. Another way to describe it is short
multiplication. And it’s a helpful way of
multiplying two-digit numbers, sometimes even three-digit numbers, by a one-digit
number, because it helps us to split these two-digit numbers into separate
parts.
We know that the number 34 is made
up of four ones and three tens. And just by writing our number like
this, we can see those tens and those ones, can’t we, in separate columns? Now, often when we’re using the
column method, we know what we need to multiply and what we need to multiply by. And we need to find the answer. But with this problem, we know the
first number of our multiplication, 34. We know what the answer is going to
be, which is 68, but we just don’t know what we’re multiplying by. Perhaps it might be a good idea to
imagine we’re going to work out this multiplication using the column method and
think about what we need to do to get from our first number to the answer.
So if we’re going to work out this
column multiplication, the first thing we do is we look at our ones digit, which is
four. And then we’d multiply it by
whatever we’re multiplying by. We don’t know. And then we’d write the answer at
the bottom. And we can see that the answer is
eight. So here’s our first clue as to what
our missing number is going to be. What do we multiply four by to get
eight? Well, we know that four doubled is
eight, isn’t it? Four times two equals eight. So it looks like our missing number
might be two. Let’s complete the digit card with
a two, and we’ll see whether it works when we multiply our tens digit.
Now in the tens place of our number
34, we have the digit three. Three 10s are 30, aren’t they? And we think that our missing
number is two. So what is 30 times two? Well, there is a smaller number
fact that we could use to help us here. We know that three times two equals
six, and so three 10s times two or 30 equals six 10s or 60. Can you see the digit six is in the
tens place in our answer? We’ve used our understanding of how
the column method works to find out the missing number. 34 times two equals 68. Our missing number is two.
