12. 【kenny2524289】来个爽点的..S为99到199的闭区间上所有odd的乘积..S中包含一个因式5的k次方..问k最大可以取值多少.. 答案:13.. no.12 quick solution: S=105 x 115 x 125 x ... x 195 = 5(21 x 23 x 25 x...x 35 x 37 x 39) because 25=5x5 and 35=5x7 so the total "5" is: 10 [from 21~39] + 3 [from 25 and 35] = 13 10 [from 21~39] + 3 [from 25 and 35] = 13 这个是什么意思啊= =不理解。。。