The probability density function of x , the lifetime of a certain type of electronic device (measured in hours), is given by f ( x ) = { 10 x 2 x > 10 0 x ≤ 10 find p { x > 20 } . what is the cumulative distribution function of x ? 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?