daily question of probability

[deepti]

there are n stations between two cities A  and B .A  train is to stop at 3 of n stations. what is probability no two of the 3 stations are consecutive.?

[Khushboo]

1- There r 2 women participating in a chess tournament. Every participant played 2 games with the other participants. The number of games that the men played between themselves proved to exceed by 66 the number of games that the men played with the women. So the number of paticipants is?

[farha]

ans:-n-2C3/nC3

[deepti]

no of participants is 13

[deepti]

in a town with a population of n,a person sends two letters to two separate persons,each of whom is asked to repeat the procedure.Thus for each letter received ,two letters are sent to separate persons chosen at random(irrrespective of what happened in the past)what is the probability that in first m stages,the person who started the chain lettr will not recieve a letter.