We used event-related potentials (ERPs) to determine the time course of mechanisms underlying strategy selection. Participants had to select the better strategy on multiplication problems (i.e., 51 x 27) to find approximate products. They could choose between rounding up and rounding down both operands to their nearest decades. Two types of problems were tested, homogeneous problems (e.g., 34 x 61) and heterogeneous problems (e.g., 61 x 36). Homogeneous problems are easier to solve because both operands are close to the lowest or the upper decades. Behavioral data revealed that participants selected the better strategy more often on homogeneous problems. ERPs showed that homogeneous problems elicited more positive cerebral activities than heterogeneous problems in the 0-200 and 800-1,000 ms windows, and more negative cerebral activities than heterogeneous problems in the 400-600 ms window. These findings have important theoretical implications for our understanding of the mechanisms underlying strategy selection.