trong 1 cuoc dua xe dap cac van dong vien phai dua tren 1 quang duong dai 18 km.biet khi nguoi thu 1 ve dich thi nguoi thu 2 cach dich 2km va nguoi thu ba cach dich 4 km .neu van toctrung binh ko thay doi thi khi nguoi thu 2 ve dich thi nguoi thu 3 con cach dich bao xa