89994 plus 7643 equals what? These look like complicatednumbers. So we could use column addition tofind the answer and end up doing a lot of exchanging. But is there anything interestingthat we can spot about these numbers that may help us to work out the answer in ourThe first number, 89994, is veryclose to a round number, a multiple of 10000. We could use this to help. And so if we add six to 89994, weget 90000. And 90000 is a much easier numberto add to mentally. But what are we going to add toIf we want the total to remain thesame, we need to subtract six from our second number. This way, the changes we’ve madewill cancel each other out. We know that 43 take away six is37. So 7643 take away six is 7637. Now all we need to do is to addthese two numbers together in our heads. 90000 plus 7637 equals 97637. Let’s check this answer using thewritten method. Four ones plus three ones equalsseven ones. Nine tens plus four tens equals 13tens. So we can write three of those tensin the tens place and exchange 10 of those tens for 100. We write the one in the hundredscolumn. Nine hundreds plus six hundredsequals 15 hundreds, not forgetting the 100 underneath, which takes us to 16hundreds. So we can write six of thosehundreds in the hundreds column and then exchange 10 hundreds for one thousand. So we can write that one in thethousands column. Nine thousand plus seven thousandequals 16 thousand. Don’t forget the one underneath, 17thousands. So we write the seven in thethousands column and then exchange 10 of those thousands for one lot of 10000. And finally, in our 10 thousandscolumn, 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 largecomplicated numbers, we could use column addition. And as we saw, we had to exchangeseveral times to find the answer. However, we noticed that 89994 wasvery close to 90000. And so by adding six to the firstnumber, we changed it into 900000, which made it a lot easier to add with. To ensure that our total stayed thesame, we took away six from the second number. And so we were left with twonumbers that we could add very quickly in our heads: 90000 and 7637. 89994 plus 7643 equals 97637.