This study was situated in a semester-long classroom teaching experiment examining prospective teachers' understanding of number concepts and operations. The purpose of this paper is to describe the learning goals, tasks, and tools used to cultivate prospective teachers' understanding of addition and subtraction with whole numbers. Research regarding children's understanding of whole number concepts and operations was used in developing learning goals, pathways for learning, and instructional tasks for the prospective teachers. Furthermore, prior research regarding prospective teachers' whole number development supported the expectation that all of the instructional tasks were reasoned solely in base-eight. Classroom episodes indicate that base-eight allows prospective teachers to reason about addition and subtraction with whole numbers in similar ways that elementary aged students' reason in base-ten.