Video: KS1-M18 • Paper 1 • Question 23

98 − _ = 28.

04:06

Video Transcript

98 take away what equals 28.

Let’s draw a bar model to help us understand what this question is asking us. This bar represents the first number in the subtraction. It’s the number that we take away something from. And it’s worth 98. This bar represents part of 98. It’s the part that we take away. But we don’t know what this bar is worth. This is the value we need to find out. Our final bar represents what’s left after we’ve worked out the subtraction. This bar is worth 28.

Another way to look at this problem is to start with 28 and work backwards. Because we’re working backwards, we need to use the inverse operation, which means we need to add instead of subtract. So instead of working out 98 take away something equals 28, we can start with 28 and think about what we add to it to make 98.

We’re working backwards and so we add instead of subtract. 28 plus what equals 98. Let’s look at how the number 28 changes as it becomes 98. We can model 28 as two tens and eight ones. And then, we can model the number 98 as nine tens and eight ones. How does this number change to get to this number? Let’s start by looking at how the ones change. 28 has eight ones. And 98 also has eight ones. We don’t need to add any ones. So we know that the number we add must end in a zero. It has no ones.

Now, let’s look at how the tens change. And they do change. We can see that they become a lot larger. 28 has two tens. And 98 has nine tens. How many tens do we add to get from two tens to nine tens? Well, two plus seven equals nine. So we must add seven tens or 70. 98 is 70 more than 28. And so we can say that 98 take away 70 equals 28.

Let’s use a place value grid and some counters just to check that our answer is correct. Here’s the number 98, nine tens and eight ones. Watch how the two digits behave as we take away our seven tens or 70. The tens digit is going to get smaller or decrease. But because we’re only taking away seven tens and no ones, watch how the ones digit stays the same.

Let’s take away 70 then, seven tens, and see whether we end up with 28. We’ll start with 98, 88, 78, 68. That’s three tens or 30 we’ve already taken away, 58, 48, 38. That’s now six tens or 60 that we’ve taken away. We’ve got to take away one more ten, 28.

And so 98 take away 70 equals 28.

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