The length of time between breakdowns of an essential piece of equipment is important in the decision of the use of auxiliary equipment. An engineer thinks that the best "model" for time between breakdowns of a generator is the exponential distribution with a mean of 15 days.
a) If the generator has just broken down, what is the probability that it will break down in the next 21 days?
b) What is the probability that the generator will operate for 30 days without a breakdown?