Answer
a)$$
p(X=x)=\left(\begin{array}{l}
2 \\
x
\end{array}\right) \times 0.95^{x} \times 0.05^{2-x}, x=0,1,2
$$
b) Because of different failure probabilities $(0.9 \text { and } 0.8$ ).
Work Step by Step
In the example $2-34,$ there are 2 devices with the probabilities of failure equal to $0.95 .$ The number of failures is then the binomial random variable with parameters $n=2, p=0.95 .$ Therefore, the probability mass function is:
$$
p(X=x)=\left(\begin{array}{l}
2 \\
x
\end{array}\right) \times 0.95^{x} \times 0.05^{2-x}, x=0,1,2
$$
In the example $2-32,$ the binomial distribution is not the right model because the 2 devices have different failure probabilities $(0.9 \text { and } 0.8$ ).
