Video Transcript
Mentally calculate seven plus 25.
This problem asks us to find the answer to seven plus 25, but to do so mentally. So we’re expected to work out the answer in our heads. While this video might show some working out on the screen, we need to remember that all of this needs to go on inside our heads. We need to calculate the answer mentally. What’s the first thought that goes through our heads when we see the calculation seven plus 25? Well, seven is a small number compared to 25. We’re adding a one-digit number and a two-digit number together.
The first thing we can remember is that when we add any numbers together, we can add them in any order. And we still get the same total. So if we swap these two numbers around, we’re starting with the larger number and then adding the smaller. So the first thing we can picture in our minds is the two numbers the other way around, 25 plus seven.
How can we add seven to 25 quickly? One thing we could do is to imagine a number line in our heads and then add on seven ones. 25, 26, 27, 28, and so on. There must be a quicker way we can mentally find out the answer. Let’s think about the number seven. We can split the number seven into five and two. The reason we’d want to do this is that we can add five really quickly mentally. 25 add five equals 30.
Now, we just need to add the two which, again, we can do really quickly cause we adding it to a multiple of 10. 30 add two is simply 32. And so we can quickly find the answer to seven plus 25, firstly, by swapping the numbers around so that we start with a larger number. And we just have to add seven to it. And then we can partition the number seven into five and two. We do this because we can add five to 25 quickly to make 30 and then add two to make 32. These are all calculations that we can do in our heads. Seven plus 25 equals 32.
