This video demonstrates two addition problem structures that students must understand to master basic facts. Each problem structure has three numbers, with one number missing. Part-part-whole number problems involve two parts combined into one whole. Join number problems involve a start quantity with a change amount that is added (or joined), resulting in a total amount. Join number problems are also referred to as change problems. Although this video illustrates that the total (or whole) is unknown in both problem structures, students may also use manipulatives to practice basic addition facts when a part or change is unknown. A solid conceptual understanding of problem structure results in the ability to transfer knowledge of basic facts to other skills such as word problem solving and computation fluency.