The paper focuses on the parallels, which are rooted in the simultaneous development of mathematics and informatics. Both mathematics and informatics are based on problem-solving. However, the approaches to determining problems, solution techniques and interpretation of results are different. The paper shows different approaches of mathematics and informatics for solving a simple problem from the informatics competition. It was presented for students, who would be future informatics teachers, and it has become the beginning of the discovery of unexpected relationships and rules' chain, the source of successive tasks, and various methods of their solution. The paper brings the results of the constructivist teaching of students in the form of a fictional interview of mathematician and informatician. Fictional cooperation of a mathematician and an informatician in analysing and solving problems will allow for a detailed analysis and comparison of both fields, which will lead to determining both common and different elements.