In a football league, the number of goals scored by the four football teams A, B, C, and D is 9, 9, 10, and X respectively. If the mode of this set of data is equal to the average, then the median of this set of data is What is the number of digits.
在某次足球联赛中，甲、乙、丙、丁四支足球队的进球数分别为9、9、10、X，若这组数据的众数与平均数相等，则这组数据的中位数是多少．
<result>9</result>