Ages and Timelines: Subtraction on the Open Number Line is one of eight units in the Contexts for Learning Mathematics Investigating Number Sense, Addition, and Subtraction (K3) This unit begins with the story of Carlos, an eight-year-old boy who is fascinated by his great-grandfathers thick, beautiful silver hair. His great-grandfather lives in Puerto Rico and Carlos is preparing to meet him for the first time. Having only seen...