Mot cano di nguoc dong thi gap mot be dang troi xuoi .Sau ki gap be 30ph thi dong co cano bi hong .Sau 15ph thi sua xong , cano lap tuc quay lai duoi theo be (van toc cua cano doi vs nuoc la ko doi) va gap lai be cach diem gap truoc do mot doan = 2,5km.Tim van toc cua dong nuoc?