Question

Seven persons are to be seated in a row. Find the probability that 2 particular persons sit next to each other.

Answer 1

Here n(S) = 7!

Let A: 2 persons sit next to each other. 

Let the 2 particular persons be together and 5 remaining people, totally 6 can be permuted in 6! ways and 2 particular persons they themselves can be permuted in 2! Ways. 

∴ the number of ways is 6! And 2!.