This lesson builds on student understanding of how to perform addition and subtraction equatiions by using the concept of math mountains.  Students will be able to find the unknown number by using the part, part, whole strategy.  This concept is foundational for second grade students to experience the connection between subtraction and addtion problems.  Students will explain their understanding by the using diagrams of math mountains.  They will explain how they deternimined the unknown by the part, part, whole system.