In decipher jumbled up description type of questions, a round-about description is given in the form of certain small relationships and direct relationship between the persons concerned is to be deciphered.
Rita told Mani, "The girl I met yesterday at thebeach was the youngest daughter of the brother-in-law of my friend's mother."
How is the girl related to Rita's friend ?
Daughter of brother-in-law -- Niece;
Mother's niece -- Cousin.
So, the girl is the cousin of Rita's friend.