Professor Stander Deviation can take one of two routes on his way home from work. On the first route, there are four railroad crossings. The probability that he will be stopped by a train at any particular one of the crossings is .1, and trains operate independently at the four crossings. The other route is longer but there are only two crossings, independent of each other, with the same stoppage probability for each as on the first route. On a particular day, Professor Deviation has a meeting scheduled at home for a certain time. Whichever route he takes, he calculates that he will be late if he is stopped by trains at least half the crossings encountered.
a. Which route should he take to minimize the probability of being late to the meeting?
b. If he tosses a fair coin to decide on a route and he is late, what is the probability that he took the four-crossing route?