It is well known that people use several strategies to accomplish most cognitive tasks. Unknown is whether they can combine two strategies. The present study found that such strategy combination can occur and improves participants' performance. Participants verified complex multiplication problems that violated the five rule (5 x 32 = 164), parity rule (5 x 12 = 65), both parity and five rules (5 x 31 = 158), or no rule (5 x 26 = 140). Participants obtained better performance on problems violating both five and parity rules than on problems violating either (or no) rule. Moreover, we found event-related potential (ERP) differences between two-rule and one-rule violation problems between 550 ms and 850 ms post-stimulus presentation, and ERP differences between parity-rule and five-rule violation problems between 850 ms and 1,400 ms. These findings have important implications to further our understanding of strategic variations in human cognition and suggest that strategy combination may occur in a wide variety of cognitive domains.