I am having alot of trouble with a homework question from my book. It asks: The time between unplanned shutdowns of a power plant has an exponential distribution with a mean of 20 days. Find the probability that the time between two unplanned shutdowns is more than 21 days. I know this much so far 1-e-(20)(?) (one minus e to the negative power of mean times any value of the continuous variable(X)) I am lost on finding X within the equation. Hope someone can help.