Then I saw the correct solution which uses stars and bars: There are \binom{15+4-1}{4-1} possible distributions for the passengers. Of all of them, only four correspond to the passengers getting off at a single stop(All at 1, All at 2...). Then the probability required would be 4/(\binom{18}{3})= 0.5% approx
In my first attempt, i said 'Well, each passenger, as a human being, chooses where to get off. So, for all to get off at stop 1, the probability would be (1/4)^15. Since there are 4 stops, the probability required would be (1/4)^14 which is approx. 0.0000003%
So, I had this problem of probability: In a bus there are 15 passenger and the bus makes 4 stops. Whats the probability that all 15 get off at the same stop?
Does anyone have a recommendation on a book on groups that teaches applications to combinatorics? I already had a course on groups, but it was full abstract