Abstract: Previous research has shown that the language complexity primarily contributed to problem difficulty in children’s solving addition and subtraction word problems, which was called language complex hypothesis. The children’s unfamiliarity with some words, phrases, sentences or linguistic forms might lead to their failures in processing linguistic description and finding solution. However, the situations described by familiar language can vary in difficulty level of representing, which implies that situation complexity could substantially contribute to problem difficulty. The present study probes into the possibility by exploring why object-separating word problems are easier to solve than object-joining word problems. There were two alternative explanations. In terms of language complexity hypothesis, the superiority of solving object-separating problems was owing to the consistent subject for all sentences described the problem. The object-joining problem didn’t have consistent subject for all sentences, thus solved with more difficulty. Since there was superiority of representing object-separating event disclosed in related research, the situation complexity hypothesis emphasized that the superiority of solving object-separating problems was owing to the superiority of representing object-separating event. Except for the problems containing key verb “give”, the problems containing key verb “take” was applied. For the latter, the object-joining problem had consistent subject, but the object-separating problem did not. According to language complex hypothesis, there should have the superiority of solving object-joining problems containing verb “take”, however, according to situation complex hypothesis, there should have the superiority of solving object-separating problems regardless of the problem types. Second graders (mean age: 8 years and four months) in a primary suburb school of Beijing city were asked to solve these problems. Results showed that there was superiority effect of processing object-separating problems regardless of the key verbs involved in the problems, which strongly supported the situation complex hypothesis. The theoretical implication on children’s mathematical cognition was discussed at the last part of this article

Key words: addition and subtraction word problems, situation representation, problem solving, language comprehension