2.NBT.B.6
Add up to four two-digit numbers using strategies based on place value and
properties of operations.
Unpacked
This standard expands students’ previous knowledge and mastery of addition to
include up to four two-digit numbers at a time. Students understand that it is possible to
add more than two numbers. They are learning that even when there are more than two
addends, they can extend the strategies they have for addition by adding all the ones
together, and all of the tens together. They are also learning that they can sometimes
make strategic choices of the order or grouping in which we add the numbers. For
example, consider the addition problem 78 + 34 + 41 + 12. Students can start by adding
78 and 34 (and it is fine if students do this, providing they do it correctly). However, it is
easier to group the 78 and 12 together, since they add up to a multiple of 10. Reordering
and regrouping the addends, we see that the sum (78 + 12) + (34 + 41) = 90 + 75 = 165.
Note: It is not expected that second graders will use the standard algorithm for adding twodigit numbers. In fact, there is no mention of the algorithm in Common Core State Standards
until 4grade (4.NBT.B.4).
In addition, students add a string of two-digit numbers (up to four numbers) by applying
place value strategies and properties of operations. Students are learning that even when
there are more than two addends, they can extend their place-value strategies for addition
by adding all the ones together, and all of the tens together. They are also
learning that they can sometimes make strategic choices of the order or grouping in
which we add the numbers. Again, it is worthy to mention that the standard algorithm is
not introduced in second grade. Students build on the strategies they have worked on
based on place value and properties of operations.
Example:
43 + 34 + 57 + 24 = __
Student 1
Associative Property
I saw the 43 and 57 and added them first, since I know 3 plus 7 equals 10. When I added them
100 was my answer. Then I added 34 and had 134. Then I added 24 and had 158.
Student 2
Place Value Strategies
I broke up all of the numbers into tens and ones. First I added the tens. 40 + 30 + 50 + 20 = 140.
Then I added the ones. 3 + 4 + 7 + 4 = 18. Then I combined the tens and ones and had 158 as my
answer.
Student 3
Place Value Strategies and Associative Property
I broke up all the numbers into tens and ones. First I added up the tens.
40 + 30 + 50 + 20. I changed the order of the numbers to make adding easier. I know that 30 plus
20 equals 50 and 50 more equals 100. Then I added the 40 and got 140.
Student 4
Then I added up the ones. 3 + 4 + 7 + 4. I changed the order of the numbers to make adding
easier. I know that 3 plus 7 equals 10 and 4 plus 4 equals 8. 10 plus 8 equals 18.
I then combined my tens and my ones. 140 plus 18 equals 158.