The present research involved gauging preschoolers' learning potential for a key arithmetic concept, the addition-subtraction inverse principle (e.g., 2+1-1=2). Sixty 4- and 5-year-old Taiwanese children from two public preschools serving low- and middle-income families participated in the training experiment. Half were randomly assigned to an experimental group; half, to a control condition. Participants were tested for an understanding of inversion before and after intervention. One-third of the 5 year olds from both groups performed at the marginally or reliably successful levels before the intervention, and three quarters of them did so in the posttest. Only one of the 4 year olds was marginally successful before the intervention and 4 year olds in the experimental group somewhat benefited from the intervention. Significant social class effect were evident.