21. During the rush period at a certain train station, every 15 minutes an express train arrives and waits 4 minutes to pick up passengers. Four minutes after the express train leaves, a local train arrives and waits 3 minutes to pick up passengers. If a passenger arrives at the station at a random time during the rush period, find the probability of each of the following. a. The express train will be waiting at the station. b. No train will be waiting at the station