. . . View This Fullsize
How would you split these lines into 2 groups?

What other lines would fit into your groups? What wouldn't fit?
Which ones do belong?
. . . View This Fullsize
Using a mental method, what's your total? How can you defend that answer?!

Number talks powerfully develop number flexibility for all students. 
. . . View This Fullsize
How would you split these numbers into 2 groups?

What other numbers would fit into your groups? What wouldn't fit?