Subtracting 1 vs. subtracting 10 | Addition and subtraction within 100 | 2nd grade | Khan Academy

• What I want you to do is pause the video and think about what 27 minus one is, and then think about what 27 minus 10 is. Alright, you might have found it pretty straightforward, but I want to think about it in terms of place value.

So let's focus on 27. So 27, I have two in the 10's place, so that two means two 10s, and we seen them here, two groups of 10, and then we have seven in the one's place, and we see those seven ones right over here.

Now I'm going to subtract one. I'm subtracting one; one is another way to think about it because this is in the one's place here, so I'm going to take away one. So let's just take away one like that, and then what am I left with?

Well, I still have my two 10s, so I could put a two in the 10's place, so I still have a two in the 10's place. And now how many ones do I have? Well, I used to have seven, I took one away, now I have six ones, so two 10s and six ones, well we call that 26.

Now let's think about 27 minus 10. So once again, this is 27, two 10s and seven ones, and now we're not going to take away any ones, but we're going to take away one 10. Let's just remind ourselves, a 10 is literally one 10.

I know that sounds like I'm saying the same thing over and over again, but I really want you to appreciate the place value. This is the 10's place, and I have a one there, so I am going to take away one 10 and zero ones. So let's take away that 10.

So let's take away, maybe I'll take this one away. I'll do it in a color that's easier to see. So I could take that away, and then what am I left with? How many 10s am I left with? Well, now I only have one full group of 10, so I have one 10, so that's the 10's place right over there.

And how many ones do I have? Well, I still have seven ones, so I still have seven in the one's place. So 27 minus 10 is one 10 and seven ones, or 17.

And another way that you could think about it, and this might be the way that you did it when I first asked you to do it. Hey, if I'm taking away a one, I'm going to reduce my one's place by one, so I'm going to go from 27 to 26.

But if I'm taking away a 10, then my ten's place is going to go down by one, so I go from a two in the ten's place to a one in the ten's place. I go from 27 to 17. The one's place didn't change.