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# Strategies for adding 2-digit numbers

CCSS.Math:

## Video transcript

so let's do a bunch of examples from the Khan Academy exercise is to to get comfortable with different ways of adding numbers so this is 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 1 less which is 77 instead of 9 there were a number that's one more that's 10 so that makes sense if you if you take away one from 78 but add that one to 9 and you're going to get the same sum it's still going to be the same thing as 78 plus 9 so this is this is legitimate this is OK right over here and it's actually might be a good thing to do because this is easier to compute because you now are adding 77 plus a nice round 10 here so you just going to increase your tens by one so it's going to be 87 a little bit easier to compute than this right over here now what do they do here add 7 & 8 so why would I add 7 tens plus 8 ones they're treating this 7 like as if it's like as if it's 7 ones so this this doesn't make sense so I would only choose this first choice let's do let's do a few more 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 5 then add 10 so they've broken up the 15 into 5 & 10 which makes sense 15 is 5 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 so this is 5 less and then they increase the second number 15 by 5 so you take away 5 from 35 you get 30 you add that 5 to 15 you get 20 yeah that makes sense too and this is also a good strategy because 30 plus 20 this three 10s plus two tens that's going to be 5 10 or 50 very easy to compute and we got that by taking a 5 from the 35 and giving it to the 15 so that makes a lot of sense let's do let's do one more select any strategy that can be used to add 41 to 50 - so select all that apply add 40 + 52 then add 1 so let's see 41 + 50 - yeah so 40 40 we've taken the one out of the 41 so we have to deal with it later 52 hasn't changed so yeah 41 + 52 is the same thing as 40 + 52 plus one we're just adding this one later and the reason why that might be easier is you can just think in terms of the tens place where as 40 + 52 is 92 then you add 1 is 93 now let's think about this one 43 + 50 C they added 2 to 41 and they took away - from 50 yeah that makes sense you you added 2 here and you took away 2 here so it's going to give us the same value it's going to get to still get us to 90 still going to get us to 93 and this is useful because it makes the it turns the 52 into a nice round number of 50 by taking 2 away from it and giving it to the 41 turning it into 43