4. The probability density function of X, the lifetime of a certain type of electronic device (measured in hours), is given by

$$f(x) = \begin{cases}

\frac{1}{10} & x > 10 \\

0 & x \le 10

\end{cases}$$

(a) Find P(X > 20).

(b) What is the cumulative distribution function of X?

(e) What is the probability that of 6 such types of devices at least 3 will function for at least 15 hours? What assumptions are you making?