The usable lifetime of a particular electronic component is known to follow an exponential distribution with a mean of 6.2 years. Let X = the usable lifetime of a randomly selected component.
(a) The proportion of these components that have a usable lifetime between 5.9 and 8.1 years is.
(b) The probability that a randomly selected component will have a usable life more than 7.5 years is.
(c) The variance of X is.