Co bao nhiêu số sáu
99999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999969999999999999999999999999999969699999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999699999999999999999999999999999999619999999999999999999999999999999999999999999999999999969999999999999999999999999999999999699999999999999999999999999999999966966999999999999999999966699999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999969999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999699999999999999999999999999999996999999999999999969999999999999999999999999999999996