Actually, 5/7 is right. If I read the question right, you can tell that only 2 consecutive numbers have a difference smaller than 2. We know that there are 6 consecutive pairs of numbers in the set, and 7C2 = 21 total pairs. So the probability that the pair isn't consecutive is 1 - 6/21 which is 15/21 or 5/7. I think you misread the question and put the negative answers too, when the question says "positive difference" .