Solving a word problem using a system of linear equations of the form Ax + By = C .

Answer:
x- large box
y small box
5x+3y=87 /×(-4)
2x+12y=105
-20x-12y=-348
2x+12y=105
-18x=-243 /÷(-18)
x=13,5
2×13,5+12y=105
12y=105-27
12y=78 /÷12
y=6,5
large box=13.5kg
small box=6.5kg