double nl , qd;
    do
    {
    printf("nhap vao luong nguyen lieu da dung(-1 de ket thuc): ");
    scanf("%f",nl);
    printf("nhap vao quang duong da di: ");
    scanf("%f",qd);
    printf("so km/l da di: %.2f\n",(double)qd/nl);
    }while(nl != -1);
    return 0;