mot dan vit troi dng bay gap mot con vit troi bay bay theo chieu nguoc lai . con vit nay cat tieng chao mot tram ban a . con vit troi dau dan lien tiep ;ban nham roi chung toi khong phai 100 ma chung toi them chung toi them 1/2 chung toi , them 1/4 chung toi va them ban nua thi chung ta moi day 100. hoi dan vit troi co bao nhieu con