8. transistors produced by one machine have a lifetime which is exponentially distributed with mean 100 hours. those produced by a second machine have an exponentially distributed lifetime with mean 200 hours. a package of 12 transistors contains 4 produced by the first machine and 8 produced by the second. let x be the lifetime of a transistor picked at random from this package. find: a) p(x 200 hours); b) e(x); c) var(x)