Video Transcript
Jackson is using place value blocks
to help him subtract 30 from 223. Finish the calculation and tell him
the answer.
In this question, we’re told that
Jackson is wanting to subtract a two-digit number from a three-digit number. He wants to find the answer to 223
subtract 30. And the first thing that we can see
about how he’s tried to work this out is that he’s written the calculation
vertically. In other words, he’s written both
numbers so that the hundreds, the tens, and the ones digits are all in the right
columns. He’s using column subtraction. Now, we’re also told that Jackson’s
using place value blocks to help him. And we can see these in this place
value grid here. Now, why does he need to use place
value blocks? Surely, he can just subtract the
ones, then the tens, then the hundreds using this column subtraction.
Well, perhaps the best way to
understand what Jackson’s done here is to try working out the answer using column
subtraction and see how far we get. First, let’s subtract the ones. 223 contains three ones. And there are no ones in 30. So we’ve got nothing to take away
here. We started off with three ones, and
we’re going to end with three ones. Now moving on to the tens column,
223 has two 10s, but we need to subtract three 10s. And we might think to ourselves,
“Well, this isn’t possible. We can’t subtract a number that’s
larger than the one we want to subtract from.”
But if we look at the two numbers
in the calculation, we can see that this has got to be possible. 223 is a lot larger than 30. So of course, we can take 30 away
from it. The problem is that we don’t have
enough tens in our tens column; that’s all. And this is where Jackson’s place
value blocks come in useful. If we look carefully at the place
value grid, we can see what he’s done here. First of all, he’s modeled his
starting number. 223 is made up of two 100s, two
10s, and three ones. And as we’ve just said, he hasn’t
had to subtract anything from his three ones. And that’s why we can see them here
in his place value grid.
But when it came to trying to
subtract three 10s from the two 10s in 223, Jackson had a problem. He doesn’t have enough 10s. What can he do about this? Well, if we look closely at his
place value grid, we can see what he’s done about this. Jackson has taken one of his 100s,
and he’s regrouped it into 10s. He knows that 10 times 10 is 100,
and so he could just exchange 100 for 10 10s blocks. Now he does have enough 10s to
subtract three 10s from. And his number is still worth
223. It’s just being partitioned
differently. Let’s go through his column
subtraction and show exactly what he’s done to help himself. So he’s got to the tens column, and
he realizes the way the number’s being partitioned at the moment. He can’t subtract three 10s.
So he takes one of his 100s. Now, instead of two 100s, he has
100. And he exchanges it for 10 10s. So instead of two 10s, in the tens
column, he now has 12 10s. Now there are different ways of
recalling this in a column subtraction, but, in this question, we’ve been given two
little boxes at the top. So we’ve completed these. So we still have the number
223. But it’s just been made up of one
100, 12 10s, and three ones. Now we can subtract those tens. 12 10s subtract three 10s leaves us
with nine 10s. And in the hundreds column, we’ve
got no 100s to take away. So this digit is going to stay the
same. But remember, instead of two 100s,
we now only have one. So 100 take away no 100s is going
to still be 100.
In this question, we found that
subtracting 30 from 223 was quite tricky because the number 223 doesn’t have enough
10s in the tens place. We looked at the place value blocks
and understood that what we needed to do was to regroup 100 into 10 10s. And by doing this, we just made the
number 223 a different way. 223 subtract 30 equals 193.
