13. In a particular city, 20% of the residents commute to work by cars, 30% of the residents commute to work by public transportations, 10% the residents uses both. Determine the probability of a resident from this city commute to work either by cars or public transportations. (A) 0.1 (B) 0.2 (C) 0.3 (D) 0.4 (E) 0.7

18. Airline overbooking is a common practice. Many people make reservations on several flights due to uncertain plans and then cancel at the last minute or simply fail to show up. Eagle Air is a small commuter airline. Its planes hold only 15 people. Past records indicate that 20% of the people making a reservation do not show up for the flight. Let X denote the number of people making a reservation but do not show up for the flight out of 15 passengers. Assume X is a binomial random variable. What of the following statement is incorrect? (A) On a given flight, the probability that all passengers have seats is $0.2^{15}$. (B) The mean number of people making a reservation but do not show up for the flight equals 3. (C) The mean number of people making a reservation and show up for the flight equals 12. (D) The standard deviation of X equals 1.549. (E) The variance of X equals 2.4.

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