Chapter 6 - Section 6.2 - Assess Your Understanding - Applying the Concepts - Page 346: 56c

$P(30)=0.01375$

$P(x)=({}_{x-1}C_{r-1})p^r(1-p)^{x-r}$ $x=30$, $r=3$, $p=\frac{2}{2+18+18}=\frac{1}{19}$ and $1-p=\frac{18}{19}$ $P(30)=({}_{29}C_{2})(\frac{1}{19})^3(\frac{18}{19})^{27}=\frac{29!}{2!\times27!}\times\frac{18^{27}}{19^{30}}=\frac{29\times28\times18^{27}}{2\times19^{30}}=0.01375$