Video: KS2-M16 • Paper 1 • Question 7

>89,994 + 7,643 = _


Video Transcript

89994 plus 7643 equals what?

These look like complicated numbers. So we could use column addition to find the answer and end up doing a lot of exchanging. But is there anything interesting that we can spot about these numbers that may help us to work out the answer in our heads?

The first number, 89994, is very close to a round number, a multiple of 10000. We could use this to help. And so if we add six to 89994, we get 90000. And 90000 is a much easier number to add to mentally. But what are we going to add to 90000?

If we want the total to remain the same, we need to subtract six from our second number. This way, the changes we’ve made will cancel each other out. We know that 43 take away six is 37. So 7643 take away six is 7637.

Now all we need to do is to add these two numbers together in our heads. 90000 plus 7637 equals 97637. Let’s check this answer using the written method. Four ones plus three ones equals seven ones. Nine tens plus four tens equals 13 tens. So we can write three of those tens in the tens place and exchange 10 of those tens for 100. We write the one in the hundreds column. Nine hundreds plus six hundreds equals 15 hundreds, not forgetting the 100 underneath, which takes us to 16 hundreds. So we can write six of those hundreds in the hundreds column and then exchange 10 hundreds for one thousand. So we can write that one in the thousands column. Nine thousand plus seven thousand equals 16 thousand. Don’t forget the one underneath, 17 thousands. So we write the seven in the thousands column and then exchange 10 of those thousands for one lot of 10000. And finally, in our 10 thousands column, we have eight lots of ten thousand and one lot of ten thousand underneath, which gives us a total of nine lots of ten thousand, 97637.

Because we’re dealing with large complicated numbers, we could use column addition. And as we saw, we had to exchange several times to find the answer. However, we noticed that 89994 was very close to 90000. And so by adding six to the first number, we changed it into 900000, which made it a lot easier to add with.

To ensure that our total stayed the same, we took away six from the second number. And so we were left with two numbers that we could add very quickly in our heads: 90000 and 7637. 89994 plus 7643 equals 97637.

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