Developing problem solving competency is one of the most important goal of Mathematics teaching. The aim of this paper is to study primary school pupils' problem solving and reasoning skills. Pupils had to solve two logical problems (formulated as multiple-choice items) and they had to write down their argumentation. A quantitative (calculating the percentage of pupils selecting each choice) and a qualitative (analyzing pupils argumentations) analysis was done. The results show that pupils

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... write an acceptable argumentation, their logical reasoning is based mainly on guessing. The research underline the dangers of using multiple-choice items for evaluating problem solving skills: pupils could select the correct answer without solving correctly the problem.