A mail-order computer business has six telephone lines. Let $X$ denote the number of lines in use at a specified time. Suppose the pmf of $X$ is as given in the accompanying table:
$p(0)=.10$, $p(1)=.15$, $p(2)=.2$, $p(3)=.25$, $p(4)=.2$, $p(5)=.06$, $p(6)=.04$
Calculate the probability of $\{\text{at least four lines are not in use}\}$.
I interpreted the problem as find $P(4 \leq X \leq 6)$ but I'm not getting the correct answer. Can someone help me see what I'm doing wrong?
Best Answer
We want $P(0 \le X\le 2)$. Remember it said not in use.