20 tracks can arranged among themselves in 20! ways.

hence n(s) = 20!

Considering the 3 tracks which played in a order as 1.

Now, total number of tracks = 18 and these can be arranged among themselves in 18! ways. n(e) = 18!.

Probability of the event is 18!/20! = 1/380.

Please correct me If I am wrong.

Thanks,

Ashish