Show that the mean lifetime of a parallel system of two components is 1 1 + 2

Show that the mean lifetime of a parallel system of two components is 1

μ1 + μ2

+ μ1

(μ1 + μ2)μ2

+ μ2

(μ1 + μ2)μ1 when the first component is exponentially distributed with mean 1/μ1 and the second is exponential with mean 1/μ2.

*30. Compute the expected system lifetime of a three-out-of-four system when the first two component lifetimes are uniform on (0, 1) and the second two are uniform on (0, 2).