How To Use The Inverse To Support Mental Subtraction

Hi, I'm Rebecca. And in this film, we're going to look at how to solve subtraction mentally using counting on. And the first step is to hope that get your children to recognise what sorts of calculations work with this strategy. And you might want to start by displaying some subtractions on a board and getting them to notice that the values of the numbers are actually very close. And because of this, calculations can be used because of the inverse relationship between addition and subtraction. So let's have a look at an example here. I have 102, take away 99. Now, this could be done with a written method, but actually the numbers are very close, so you can just count on. You can even use your fingers. 99, 100, 101, 102. So the answer is three. Looking at a slightly more complicated 1, 1005, take away 989 is a bit further away, but it's close enough to use counting on. And we can do this with a number line because it bridges the gap between the mental calculation and the written method. So I'm going to start by doing a jump. I'm going to do a jump of 11, which will take me to 1000. Then I can see that I'm five away from 1005, so I will do a jump of five. And then I can see that I've needed to count on 16 to get myself to 1,005. So my answer is 16. Looking at a slightly more complicated one with bigger numbers, this is even more important because lots of pupils think that where the numbers are very big, the gaps between them are big too, and they end up doing unnecessary and inefficient calculations. But actually, if you look at this one, you can see that it's only 4 in difference. So the answer to this one is Thor. One way to get this going in your classroom is to have a race. You could have some children doing it by counting on and some children doing it with a formal written method and see who gets there first. So there you go, that's counting on for subtraction.