Average number of customers arriving in 3 minutes is: $\frac{25}{30}\times 3=2.5$ $P(X=x)=\frac{e ^{-\lambda}\lambda ^{x}}{x!}$ $P(X=0)=\frac{e ^{-2.5}2.5^{0}}{0!}=e ^{-2.5}$