Question: who are bilbo and frodo?
Answer: Bilbo is the son of Bungo (son of Mungo). Frodo is the son of Drogo, who is the son of Fosco (son of Largo, Mungo's younger brother). Therefore Bilbo and Frodo share a common ancestor in Balbo, father of Largo and Mungo. Since that makes Bilbo and Drogo second cousins, Frodo is Bilbo's second cousin once removed.