Subtracting by counting backwards
- [Instructor] Let's see if we can think about what number is equal to 13 minus four, and to help us think about that, let's look at 13 blocks here. And the way that it's been set up, you can see that 10 have been arranged in this bar over here. One, two, three, four, five, six, seven, eight, nine, 10 and that makes sense because 13 is one 10 and three ones. One 10 and one, two, three ones. So this right over here is a total of 13 blocks. Now, if we're subtracting four, we're gonna take four of those blocks away. So let's do that. We'll take away this one, that's one, two, three, and then we'd have to take one away and break this group of 10. And so, how many do we have left? Well, we can see very clearly we have one, two, three, four, five, six, seven, eight, nine left and so we could write a nine right over there. Another way to think about it is if you take away three, you're just left with this bar of 10 and then you have take away one more and then you are left with nine.