applied probability

After the person told the rumour, there are 3 persons who heard about it and 13 person who had not.
There is an uncertainty in the question.
In common sense, we have to make the assumption here: the 2 persons will not tell back to the initiator as well as to each other, i.e. they have only 13 people to choose from to tell the rumour.
There are 13C2 = 78 ways for one person to choose his recipient.
There are in total 78 * 78 = 6084 ways for the 2 persons to choose the recipients.
There are 13C4 = 715 ways to choose 4 different persons so there will be no repetition.
Therefore the probability of no person hears it twice
= 715 / 6084
= 55 / 468
= 0.1175
If the the 2 persons simply blindly choose the recipients, each has 15C2 = 105 choices.
Total number of ways for the two to choose the recipients
= 105 * 105 = 11025
The number of ways that the 4 people chosen will hear the rumour only once is again 13C4 = 715
Under such situation the required probability
= 715 / 11025
= 0.06485