Collaborative learning is a subject that has occupied many researchers throughout the world. Many researchers have maintained that when students of all school stages, from kindergarten to higher education work in teams, this leads to high performance (Johnson, Skon, & Johnson, 1980), but also has a positive impact emotionally and psychologically (Schmitz & Winskel, 2008). However, there have been studies claiming that teamwork does not actually result in any substantive improvement in students’ efficiency (Samuelsson, 2010), or that only some students gain from this method of learning (Sears & Reagin, 2013). The present study explores how cognitive development relates to problem solving in pairs or individually, in students of the sixth grade in school using the numerical puzzle Sudoku. More specifically, the study explored four different types of pairs of two, according to the level of cognitive development (high – high, high – low, average – average and low – low) and three different types of units (high, average, low), all taken from sixth graders of a public elementary school. Additionally, the research studied whether solving Sudoku was more effective in pairs rather than individually. The sample of this research included thirteen sixth grade classes from a public elementary school in a city in Cyprus. The city was intentionally selected to facilitate the research. Two hundred twenty students completed the tool of cognitive development Group Assessment of Logical Thinking (GALT). Then, 60 students individually solved the Sudoku puzzle, while 160 students solved the puzzle in pairs. The results of the qualitative study showed that there were differences between the four types of pairs in terms of how they collaborated. Although students showed interest or even enthusiasm in solving the Sudoku puzzle, the majority of them did not work together so as to improve their performance. The results of the quantitative study confirmed that students failed to cooperate. The performance of the Individual Problem Solving Ability was not different among the four types of pairs. The majority of high-level cognitive development pairs started to solve the puzzle competitively. However, in the process they worked together to solve the puzzle correctly. They also behaved in an intensely self-centred way. In non-homogeneous pairs, low-performing students had a passive role in the group. Additionally, it seemed that the level of cooperation was related to high performance. The research demonstrated that students had difficulty in developing combinational thinking. This was the reason they could not solve the puzzle. Pairs did not cooperate, despite the fact that they had clear guidelines to do that. This finding should be a concern for teachers and the educational system of Cyprus, in general. The role of the teacher should be supportive in helping students overcome their difficulties, considering the theory of Vygotsky (2012) on systematic facilitating, development, and the Zone of Proximal Development.