Every time you fill a box, you increase the number of empty boxes by 7 (because you add 8 empty ones and fill one) and the total number of boxes by 8.
102-11=91 gives the number of extra empty boxes you will need.
91/7= 13 gives number of boxes that must be filled to achieve this
13*8= 104 gives extra boxes required.
104+11 = 115 is the total number of boxes