There are 10 stations on a railway line. The number of different journey tickets that are require by the authorities , is

There are 10 stations on the railway line. A journey ticket is for traveling between two different stations, where the order matters (i.e., traveling from station A to B is different from traveling from B to A).

For each starting station, there are 9 possible destinations (since you can't travel to the same station). Since there are 10 possible starting stations, the total number of different journey tickets is $$10\times9=90$$