4.11. Consider a satellite whose work is based on a certain block A. This block has an independent backup B. The satellite performs its task until both A and B fail. The lifetimes of A and B are exponentially distributed with the mean lifetime of 10 years. (a) What is the probability that the satellite will work for more than 10 years? (b) Compute the expected lifetime of the satellite.

4.11. Consider a satellite whose work is based on a certain block A. This block has an independent
backup B. The satellite performs its task until both A and B fail. The lifetimes of A and B
are exponentially distributed with the mean lifetime of 10 years.
(a) What is the probability that the satellite will work for more than 10 years?
(b) Compute the expected lifetime of the satellite.