Video: KS2-M17 • Paper 1 • Question 13

_ − 100 = 1,059.

02:09

Video Transcript

What minus 100 equals 1059?

To fill in the gap in this number sentence, we need to think backwards. We’re given the answer to the number sentence. And we’re told what’s happened to the first number to get to the answer. But we need to work out what that first number is. So we need to work backwards using the information we’ve been given.

If we’re not sure which operation to use and how to find the missing number, often sketching a bar model is a quick way to help us visualize the problem. We start off with a number we don’t know, we take away 100, and we’re left with 1059. So we can see just by looking at our quick bar model that to find the missing number we need to add 1059 and 100 together, in other words, what is 100 more than 1059.

When we add 100 to a number, the hundreds digit increases by one unless of courses it is a nine, in which case we move on to the next thousand. But in this example, we have zero hundreds. So if we add 100, our hundreds digit will be a one, 1159.

And so because we knew the final number in the number sentence, we were able to work backwards and use the opposite operation to find out the missing first number. We imagine our number machine that takes away 100 from every number that goes into it. We know the number that comes out. We need to work out the number that went in. We use the inverse operation, the opposite operation. And that’s why we had to find 100 more than 1059.

So what take away 100 equals 1059? 1159 take away 100 equals 1059.

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