Question Video: Using Place Value to Subtract a One-Digit Number from a Two Digit Number without Regrouping Mathematics • 1st Grade

Use place value to subtract numbers. Pick the correct way to break apart 47 into tens and ones. Subtract 5 from 47 by taking away 5 ones.


Video Transcript

Use place value to subtract numbers. Pick the correct way to break apart 47 into tens and ones. Subtract five from 47 by taking away five ones.

The main calculation that we need to do in this question comes right at the end. We need to subtract five from 47. But before we get to this step, there’s one or two things we need to do first. The first sentence tells us that we need to use place value to subtract numbers. Do you remember what these words “place value” mean? They’re all about how the digits in a number can have different values depending on where you write them, depending on their place in a number. And we can see, just by quickly looking at this question, we’re thinking about two-digit numbers.

In the first part of the question, we’re asked to start thinking about a two-digit number. We need to pick the correct way to break apart 47 into tens and ones. We know from later on that 47 is one of the numbers in our subtraction. And it looks like it’s going to be helpful to us to split up this number into its tens and ones. The diagram shows that two different place value grids. And each one shows a different number of tens and ones. But which one of the two is correct, seven 10s and four ones or four 10s and seven ones?

Hopefully, you can hear the similarities as we read those out. They both contain seven of something and four of something else. In other words, they both represent numbers that contain a seven digit and a four digit. But which way around?

To answer this part of the question, we need to understand something about the digits in 47 and their place value. And to help us, we could write those digits into a place value grid. Our grid shows two spaces, one for the tens and one for the ones. And we can write our digits in order. The digit that comes first is a four, and that’s going to go in the tens place. So 47 has four 10s. We know this because four 10s are worth 40. And there’s only one place for us to write our digit seven, and that’s in the ones place. This has a value of seven ones.

Now, we can use this to help us find the correct answer. We’re looking for the place value grid that shows four 10s and seven ones. That’s this one here. The other model shows seven 10s and four ones. It’s the number 74. But we know that the number our subtraction begins with can be modeled using four 10s and seven ones.

In the final part of the question then, we’re asked to subtract five from 47 by taking away five ones. Can you see how the subtraction has been written in a place value table for us? Now, the calculation could’ve been written like this. But by writing the numbers in a place value table like this, we’re really doing what we’ve done in the first part of the question. We’re splitting them up into their tens and their ones. We’re using place value to help us.

Now, we’re told we need to find the answer by taking away five ones. And this is because the number that we need to subtract, five, is worth five ones. If you look in our place value table, can you see the digit five in the ones place? These are the five ones we need to take away. So why does this question ask us to take away a number of ones? Why doesn’t it say a number of tens and some ones?

Well, that’s because we’re only taking away a one-digit number. The number five doesn’t have any tens in it. We just need to subtract a number of ones. And in the number 47, we already have seven ones. So we know we’ve got enough ones to take away our five ones without doing anything else. Let’s cross out five of our ones blocks and see what we have left. So we’ve got seven to start with, and then six, five, four, three, two. Let’s write what we have left in our place value grid. We had seven ones, we’ve taken away five ones, and now we’re left with two ones. And as we’ve said already, we don’t need to do anything with the tens.

We started off with four 10s. We don’t need to take any tens. And four take away zero leaves us with the same number. We still have four 10s. Our answer contains four 10s and two ones. It’s the number 42.

In this question, we needed to subtract five from 47. The first thing we were asked to do was to break 47 apart into its tens and ones. And we know 47 has four 10s and seven ones. And by modeling it using place value blocks, we could then take away five ones blocks. And that’s how we know 47 take away five equals 42.

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