Participants were asked to verify if complex additions were smaller than 100 or not. Two hundred and forty arithmetic problems were presented, with half the problems being small-split problems (i.e. proposed sums were 2 or 5% away from 100) and half being large-split problems (i.e. proposed sums were 10 or 15% away from 100). Behavioral and ERPs data indicate that participants may use two different strategies to verify complex inequalities, a whole-calculation strategy for small-split problems and an approximate-calculation strategy for large-split problems. The choice between these two strategies occured within 250 ms post-stimulus presentation, and strategy execution was lateralized. Implications for our understanding of the brain mechanisms underlying arithmetic problem solving are discussed. (C) 2002 Elsevier Science Ltd. All rights reserved.