pair

1

The ordered pair $(a,b)$ can be chosen in 16 ways so as not to be a multiple of $(c,d)$.

There cannot be two edges between one pair of vertices.

2

Write out the integers from 1 to $n$. Pair up the first and the last, the second and next to last, etc.

Go to the list of words starting with: a b c d e f g h i j k l m n o p q r s t u v w y z