This study investigates student understanding of the part-whole concept in mathematics at an independent two-year post-secondary institution in New Mexico. The mathematics portion of the Tests of Adult Basic Education (TABE) Summary Form was used as the evaluative instrument. Evidence for a lack of part-whole concept understanding as a hidden cause of innumeracy is reported. Data shows a trend of a higher final grade corresponding to better part-whole understanding at the start of the course. In light of this evidence, it is concluded that direct instruction of the part-whole concept should be part of adult basic math and algebra classes. (SOE)