Two different numbers are selected simultaneously and at random from the set {1,2,3,4,5,6,7}. What is the probability that the positive difference between the two numbers is 2 or less? Express your answer as a common fraction.
1 2 3 4 5 6 7
1 1 2 3 4 5 6
2 1 2 3 4 5
3 1 2 3 4
4 1 2 3
5 1 2
6 1
P (diff 2 or less) = 11 / 21
