Answer:
Number of containers they used is 11.
Explanation:
Assuming all the containers will be filled to the maximum level we can send 96 chocolate boxes in the following ways,
1) First way is to send 12 containers of large chocolate boxes.(

But this is not viable because it says that they have sent both small and large boxes.
2) Second way is to send 2 containers of large chocolate boxes
and 8 containers of small chocolate boxes
. So the total will be
.
But this is not possible as they have sent more large chocolate boxes than small chocolate boxes.
3)Third and final way is to send 7 containers of large chocolate boxes
and 4 small chocolate box containers
. So the total number of chocolate boxes is
.
In this case they have sent more from the large chocolate boxes than small chocolate boxes.
So the total number of containers they used is
.