Strategies for adding 2-digit numbers | 2nd grade | Khan Academy

• So let's do a bunch of examples from the Khan Academy Exercises to get comfortable with different ways of adding numbers.

So this says, select any strategy that can be used to add 78 plus 9. Select all that apply.

So this first choice is 77 plus 10. Well, does that make sense? So instead of 78 they wrote a number one less which is 77, and instead of nine they wrote a number which is one more, that's 10. So that makes sense. If you take away one from 78 but add that one to nine, you're going to get the same sum. It's still going to be the same thing as 78 plus nine. So this is legitimate; this is okay right over here.

And this actually might be a good thing to do because this is easier to compute. You now are adding 77 plus a nice, round 10 here. So you're just going to increase your 10s by one, so this is going to be 87. A little bit easier to compute than this, right over here.

Now what do they do here? Add seven and eight. So why would I add seven 10s plus eight ones? They're treating the seven like as if it's seven ones, so this doesn't make sense. So I would only choose this first choice.

Let's do a few more examples. Select any strategy that can be used to add 35 plus 15. Select all that apply.

So here they say add 35 plus five then add 10, so they've broken up the fifteen into five and 10 which makes sense. Fifteen is five plus 10. And this is a decent strategy because this gets us to 40, which we can do in our head, and then we add 10, that gets us to 50. So that one definitely works out.

Now what are they doing over here? 30 plus 20. So instead of writing 35, they wrote 30. This is five less, and then they increased the second number 15 by five. So you take away five from 35, you get 30. You add that five to 15, you get 20. Yeah, that makes sense too, and this is also a good strategy because 30 plus 20, that's three 10s plus two 10s; that's going to be five 10s or 50. Very easy to compute, and we got that by taking a five from the 35 and giving it to the 15. So that makes a lot of sense.

Let's do one more. Select any strategy that can be used to add 41 to 52. So, select all that apply.

Add 40 plus 52, then add one. So let's see, 41 plus 52, yeah so 40, we've taken the one out of the 41, so we're going to have to deal with it later. 52 hasn't changed, so yeah, 41 plus 52 is the same thing as 40 plus 52, plus one. We're just adding this one later, and the reason why that might be easier is you could just think in terms of the 10's place first. 40 plus 52 is 92, then you add one; it is 93.

Now let's think about this one. 43 plus 50. See, they added two to 41 and they took away two from 50. Yeah, that makes sense. You added two here and you took away two here, so that's going to give us the same value. It's still going to get us to 93, and this is useful because it turns the 52 into a nice, round number of 50 by taking two away from it and giving it to the 41, turning it into 43.