Answer is 2..
Average is 7 x 3 = 21
Median is 9, so the sum of the other two box = 12! (21-9)
Now, the heavier box shud be more than 9 (as 9 is the median)
Assuming all weights are INTEGERs!
The min weight of the hevier box wud be 9+1 = 10!
Hence the max weight of the lightest box = 12-10 =2!
Why can't it be equal to 9?
"To dream anything that you want to dream, that is the beauty of the human mind. To do anything that you want to do, that is the strength of the human will. To trust yourself, to test your limits, that is the courage to succeed."
- Bernard Edmonds