Statistics and probability question
frankie rides the subway to work. she either takes the local or the express, whichever comes first. the local arrives every 15 minutes, at 00, 0: 15, 0: 30, and 0: 45 past the hours. the express arrives every 20 minutes at : 05, 0: 25, and 0: 45 past the hours (if they both arrive at the same time she takes the express). assuming that she arrives the subway station at a random time, what's the probability that she will take the express to work.