A Girl plans to invite 15 friends to dinner. For 35 days she wants to
have dinner with exactly three friends a day, and she wants to arrange
the triplets in such a way that each pair of friends will come only
once (e.x., A and B can only appear once, though A or B can dinner with the girl and the others as many as you like). Please help the girl to make the arrangement.

Are there any patterns to solve this type of question?