Solution

Let 

$A_i$ represent that $i^{th}$ alarm is activated when temperature reaches $135^\circ F$.

Given: $P(A_i)=0.8~ \forall~ i \in \{1,2,3\}$

b. Probability that the alarm will function when temperature reaches F135∘F is given by P(x ≥1).

P(x ≥1)​=1−P(x=0)

=1−(0.2)3

=0.992

$Answer$

b. $P(x \ge 1)= 0.992$

c. $\mu=2.4, ~ \sigma^2=0.48$