Time to failure in months, X
Fx(x) = (1 - e ^(-x/5))u(x) for plant A
Fx(x) = (1 - e ^(-x/2))u(x) for plant B
Plant B produces three times as much as plant A.
What is the probability that a bulb purchased at random will burn at least (a) two months, (b) five months and (c) seven months