Example A certain type of magnetic disk must function in an environment where it is
exposed to corrosive gases. It is known that 10% of all such disks have
lifetimes less than or equal to 100 hours, 50% have lifetimes greater than 100
hours but less than or equal to 500 hours and 40% have lifetimes greater than
500 hours. Let Z represent the number of hours in the lifetimes of a randomly
chosen disk
a) Find P(Z<=500)