You could say that one large box is x and one small box is y.
So, 5x+6y=187
3x+2y=87
You could first solve for x (large box)
Find the LCM of 6 and 2 (6)
5x+6y=187
9x+6y=261
261-187=74
4x=74
74/4=18.5(for large box)
Now for the small box.
Find the LCM of 3 and 5 (15)
15x+18y=561
15x+10y=435
561-435=126
8y=126
126/8=15.75(small box)
(all answers are in kg)