We describe, at the microscopic level, the dynamics of N interacting components where the probability is very small when N is large that a given component interact more than once, directly or indirectly, up to time t, with any other component. Due to this fact, we can consider, at the macroscopic level, the quadratic system of differential equations associated with the interaction and establish an explicit formula for the solution of this system. We moreover apply our results to some models of Econophysics.