Mot dan vit troi dang bay thi co 1 con bay theo chieu nguoc lai. Con nay len tieng chao: " Chao 100 ban a ! ". Con vit troi dau dan lien dap: Ban nham roi ! Chung toi khong phai mot tram, ma chung toi them chung toi, them 1/2 chung toi, them 1/4 chung toi va them ca ban thi chung ta moi day 100 con. Hoi dan vit troi co bao nhieu con ?