Suppose that a random variable $X$ has a probability density function
\[
\begin{aligned}
f(x) & =c(x-4) \quad \text { for } 4 \leq x \leq 6 \\
& =0 \quad \text { for all other } x
\end{aligned}
\]
for some constant $c$. What is the expected value of $X$ given that $X \geq 5?$
- $5 \frac{5}{9}$
- $5 \frac{1}{2}$
- $5 \frac{3}{4}$
- $5 \frac{1}{4}$
- $5 \frac{5}{8}$