An electronic chess game has a useful life that is exponential with a mean of 30 months. Determine each of the following: a. The probability that any given unit will operate for at least (1) 39 months, (2) 48 months, (3) 60 months. b. The probability that any given unit will fail sooner than (1) 33 months, (2) 15 months, (3) 6 months. c. The length of service time after which the percentage of failed units will approximately equal (1) 50 percent, (2) 85 percent, (3) 95 percent, (4) 99 percent.