Xiaoqiang puts 99 marbles into two kinds of boxes, each big box can hold 12 marbles, and each small box can hold 5 marbles. If there are more large boxes than small boxes, and each box is exactly full, find the number of large boxes used.
小强把99颗弹子放进两种盒子，每个大盒可装12颗弹子，小盒可装5颗弹子．假如大盒的数目比小盒的数目多，而且每个盒子都刚好装满，求所用大盒子的数目．
<result>7</result>