Question: 1. Each domino piece is marked by two numbers. The pieces are symmetrical so that the number-pair is not ordered. How many different pieces can be made using the numbers 1, 2, ... , n?
2. The numbers 1, 2, ... , n are arranged in random order. Find the probability that the digits
(a) 1 and 2,
(b) 1, 2, and 3, appear as neighbors in the order named.