Video: KS2-M16 • Paper 2 • Question 3

Write the three missing digits to make this addition correct.

03:10

Video Transcript

Write the three missing digits to make this addition correct.

And we can see that the question shows us a vertical addition, showing the calculation one hundred and fifty something add four hundred and something four equals something hundred and fifteen. We have three missing digits. We need to work out what they are.

When we add or subtract numbers vertically, we always start with the ones column first and move from right to left. So let’s start by thinking about this missing digit in the ones column and go from right to left in this problem too. What do we know about this first missing digit?

Well, we know that there are two possibilities. Either when we add four to it, we get five or when we add four to it, we get 15. Both of these calculations would give us the five ones that we can see in the answer. But which one should we use: one plus four equals five or 11 plus four equals 15?

Well, we know that, in the ones column, we can only write one single digit. And so the answer must be the first one. Let’s think about the tens column. What do we add to five tens to give us the answer one ten? Does this make sense? Of course, one ten is less than five tens. So what must have happened is a number of tens must have been added to five tens to make 11 tens.

Let’s exchange 10 of those 11 tens for 100 and write a one in the hundreds column. What do we add to five tens to make 11 tens? Five tens plus six tens equals 11 tens. So we now know our second digit. And we also know the whole calculation, 151 plus 464.

Let’s finish the problem by looking at the hundreds column: 100 plus 400. We might think the answer is 500. But don’t forget we exchanged 10 tens for 100, so we have that to remember as well. 100 plus 400 plus the one extra 100 equals 600.

Let’s quickly go through the addition to make sure that we’re correct. One one plus four ones equals five ones. Five tens plus six tens equals 11 tens. And we exchanged 10 of those tens for 100. And finally, 100 plus 400 plus the 100 we’ve just exchanged equals 600. And so our three missing digits from top to bottom are one, six, and six.

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