Adding 1 vs. adding 10 | Addition and subtraction | 1st grade | Khan Academy

So pause this video and real quick figure out what 27 plus 1 is, and then if possible, figure out what 27 plus 10 is.

All right, so a lot of, let's think about it together. You might have been able to do this one pretty easily. You might have said, okay, if I'm adding one, it's just going to be the next highest number from 27. I would go to 28 if I have one more. But the reason why I'm doing this exercise is to really think about place value.

So over here, the number 27, we have a 2 in the tens place. This is in the tens place right over here, and then we have a 7 in the ones place. So you could visualize the number 27 as 2 tens, and I have two groups of ten here, and then seven ones, and I have seven ones right over here.

And then if I add one more, that's one more of these individual ones. And so what am I going to be left with? Well, I'm still going to have two tens. I'm still going to have two tens, but how many ones am I going to have now? Oh, one, two, three, four, five, six, seven, eight ones. So it's going to be 28.

So that might not be surprising for you, but it's really valuable to think about in terms of place value because it's going to be very useful later on as you learn more math. Now let's do the same thing with 27 plus 10. We already said the 27; that's two tens and seven ones.

And now with ten, the one isn't in the ones place, so it doesn't just represent one. The one here is in the tens place, so we have one ten and we have zero ones. So we just have the number 10—it's just a group of ten here.

So when you add these together, what are you going to have? Well, now you're going to have one, two, three tens. So you're going to have 3 in the tens place, and then you're still going to have, you're still going to have one, two, three, four, five, six, seven ones—seven ones.

So notice here when you added a one, you increased your ones place by one, and here when you added 10, which is literally one ten, you didn't increase the ones place; you increased the tens place.