Birthday problem

Had been to Corner House, Residency road today.(Vanilla Ice cream with forrest berry sauce is just too good). Two different people were celebrating their birthdays. A world is too big a place to talk as an example. In a sample size of say, 50, if you want to find out how many share your birthday, then, use simple probability principles and voila! a mathematical tool to address your problem! probability theory (.pdf) does leave me dumbfounded..