How many equivalence relations on $\{1, 2, 3\}$ contain the pair $(1, 2)$?

Explanation

1. 01

   1 and 2 must share a class; 3 either joins them or stands alone.

2. 02

   $\{1, 2, 3\}$ or $\{1, 2\}, \{3\}$: 2 relations.