This presentation investigates subtraction as an unknown addend problem: Common Core 1.OA.4. This PowerPoint helps students visualize turning a subtraction problem into an unknown addend addition problem and solving using a number line. Several examples are shown, using all numbers and then using an unknown. Then each subsequent slide presents a different problem with pauses for the students to answer. This lesson is an overview of 1.OA.4, and can be used repeatedly as a lesson with white board, choral or individual response. THIS IS AN ADDITIONAL LESSON AND DIFFERENT FROM THE ONE ALREADY OFFERED.