The probability density function of X, the lifetime of a certain type of electronic device (measured in hours), is given by $$f ( x ) = \left{ \begin{array} { l l } { \frac { 10 } { x ^ { 2 } } } & { x > 10 } \ { 0 } & { x \leq 10 } \end{array} \right.$$ (a) Find P{X>20}. (b) What is the cumulative distribution function of X? (c) 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? (d) Let Y be the lifetime of the same device measured in minutes. What is the probability density function of Y ?