The following forty-one numbers 55...□99...9 (5 and 9 each have 20) can be divisible by 7, so what is the number in the middle square?
下面这个四十一位数55…□99…9(其中5和9各有20个)能被7整除，那么中间方格内的数字是多少．
<result>6</result>