Let $P_1, P_2$ and $P_3$ are the probabilities of a student passing three independent exams $A, B$ and $C$ respectively. If $P_1, P_2$ and $P_3$ are the roots of equation $20x^3-27x^2+14x-2=0$, then the probability that the student passes in exactly one of $A, B$ and $C$ is