In a network of railways, a small island has 15 stations. The number of different types of tickets to be printed for each class, if every station must have tickets for other station, is

For each pair of stations, two different types of tickets are required, Now, the number of selections of 2 stations from 15 stations$=15\times 14$.

$\therefore$ Required number of types of tickets $=15\times 14=210$