In the experience described in this paper, pupils in 3rd and 4th year primary school were asked to organize a self-financed party by holding an auction of all useful objects no longer used at their own homes, both events to be open to all the parents. The activity, although presented in a brief, clear “announcement”, needs to be broken down into smaller sub-problems (to be dealt with singly) and offered an opportunity to face, without the intervention of any theoretical background, questions connected to early operations with decimal numbers and ratios and proportions. Moreover, the complex situation required the children to recognize and acquire some of the key skills needed for active participation in social life, such as how to solve a problem collectively.