“Ages and Timelines: Subtraction on the Open Number Line” is a study guide published by Heinemann in 2008 as part of the Context for Learning Math Series. Written by Catherine Twomey Fosnot, the book focuses on teaching subtraction using the open number line, making it suitable for both high school and elementary school students. The trade paperback format includes 76 pages filled with information on counting, numeration, arithmetic, and teaching methods. This educational resource provides valuable insights for homeschooling and traditional classroom settings.