The time between arrivals at an airport check in counter is exponentially distributed with a mean of 4 minutes. (a) What is the probability that the person working the counter waits longer than 20 minutes before a passenger arrives? (b) Suppose the person working the counter has already been waiting for a passenger for 30 minutes, what is the probability the next passenger will arrive within the next 10 minutes? (c) Determine x such that, the probability of waiting longer than x minutes for the next passenger to arrive, is equal to 25%