Research findings have established that students often struggle with mathematical symbols including common misconceptions for literal symbolic representations of variables but provide little evidence of when or how these misconceptions arise. This article reports findings from a study of grade 4-6 students' conception(s) for various representations of unknown addends [variables] commonly found in US elementary mathematics textbooks. Specifically, thirty-six US grade 4-6 students participated in two semi-structured task based interviews designed to explore their conception(s) of formal and informal representations of unknown addends as revealed by the types of numbers substituted across core mathematical tasks and task types. Findings showed that participants initially demonstrated a natural number bias and upon further questioning a whole number bias. Participants' number substitutions did not include common established difficulties for literal symbols exhibited by students in algebra courses and higher-level mathematics courses. In addition, even when providing the same types of number substitutions, participants attended to various attributes of the different representations for unknown addends.