the given answer is 5/72. My answer is since there are 4 people so " _ _ _ _ ". if put them in paires then in the first paire the first person has a choose of 6 stops to choose from but the second person have to get off at the same stop, so the out come is 6*1. For the second pair, similarly, it's 5*1. So total has 6*1*5*1 = 30 possible out comes, then the P is 30/6^4 which desnot equal to 5/72. I don't understand how they come up with the 5/72 answer.

Thanks.