Ages and Timelines: Subtraction on the Open Number Line

This curricular unit in the Contexts for Learning Mathematics series explores various subtraction models (difference, removal, missing addends) and relates addition to subtraction while developing mental arithmetic strategies for subtraction with 2- and 3-digit numbers. Children interview their family members and compare age differences. Timelines are introduced as a context for using the open number line - a helpful model used as a tool to explore and represent strategies for addition and subtraction. This unit will focus on the open number line as a model for subtraction. In contrast to a number line with counting numbers written below, an “open” number line is just an empty line used to record children's addition and subtraction strategies. Only the numbers that children use are recorded and the addition and subtraction are recorded as leaps or jumps. As the unit progresses, timelines are used to record years of birth, rather than ages. This change in context challenges learners to grapple with larger numbers and with the changing places of the part-whole relations of numbers on the number line. For example, first the number 79 may be marked on the number line as 8 less than 87; then it may be the difference between 2005 and 1926. Several minilessons for subtraction are also included in the unit. These are structured using strings of related problems as a way to guide learners more explicitly toward computational fluency with subtraction. To learn more visit https://newperspectivesonlearning.com Read more