The problem is:
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.
$$\begin{array}{c|c|c}
x & \text{0} & \text{1} & \text{2} & \text{3} & \text{4} & \text{5} & \text{6}\\
\hline
\\p(x) & .10 & .15& .20 & .25 & .20 & .06 & .04
\end{array}$$Calculate the probability of each of the following events.
a. {at most three lines are in use}
b. {fewer than three lines are in use}
c. {at least three lines are in use}
d. {between two and five lines, inclusive, are in use}
e. {between two and four lines, inclusive, are not in use}
f. {at least four lines are not in use}
The only parts I was unable to do were parts e) and f).
For part f), I removed the negation of the statement, hoping that that might make the original statement more clear. So, if we say that we want the probability of the at least being in use, that would be the probability of $4$ in use, $5$ in use, or $6$ in use. But we don't care to know that probability, we want to know the opposite: we want to know the probability of less than 4 lines in use. This process didn't get me the right answer, however.
Could someone help me with those two parts?
Best Answer
Isn't f the same as b? For a, you are counting if $0,1,2,$ or $3$ lines are in use, so just add $0.10+0.15+0.20+0.25=0.70$ The rest are similar.