Answer:
The weight of a small box  =  13.5 kg
The weight of 1 large box  = 18.5 kg
Step-by-step explanation:
Let us assume the weight of a small box = m kg
And the weight of 1 large box  = n kg
Now, the weight of 5 small box = 5 x (weight of 1 small box)  =  5 m
Also, the weight of 3 large box = 3 x (weight of 1 large box)  =  3 n
Here,   3 large boxes +  5 small boxes =  of 123 kilograms
⇒ 5  m + 3 n = 123   .... (1)
Again, the weight of 2 small box = 2 x (weight of 1 small box)  =  2 m
Also, the weight of 12 large box = 12 x (weight of 1 large box)  =  12 n
Here,   12 large boxes +  2 small boxes =  249 kilograms
⇒ 2 m+ 12 n = 249   .... (2)
Now, solving both the given equations by ELIMINATION, we get:
 5  m + 3 n = 123     x (2)
 2 m+ 12 n = 249     x (-5)
we get the new set of equitation as:
10 m + 6 n = 246
- 10 m - 60 n =  -1245
Adding both equation, we get
-54 n = 999
or, n = 18.5
Now, 5  m + 3 n = 123
So, 5 m = 123  -3 (18.5)  = 123 - 55.5 =  67.5
⇒  m = 13.5
Hence,  the weight of a small box = m kg = 13.5 kg
And the weight of 1 large box  = n kg  = 18.5 kg