This report is frammed with the process of design, implementation and validation of a teaching situation of conditional probability. It draws upon the game Monty Hall as scenary for decision-making in face of situations of uncertainty. In particular, under the didactic engineering framework, preliminary analysis elements are given. Cognitively, students use non mathematic elements to decide. Epistemologic analysis focuses on the exchange of correspondence about gamblings between Pascal and Fermat. The didactic elements show a prevalence of probabilistic calculations over decision making in textbooks.