11. The profit is $350 for each computer assembled by a certain person. Suppose that the assembler guarantees his computers for one year and the time between two failures of a computer is exponential with mean 18 months. If it costs the assembler $40 to repair a failed computer, what is the expected profit per computer?

Hint: Let N(t) be the number of times that the computer  fails in [0, t]. Then N(t) : t ≥ 0

is a Poisson process with parameter λ = 1/18.