Answer:
both size boxes are $18 each
Explanation:
Let x and y represent the costs of the small and large boxes, respectively. Then the two sales are ...
3x +14y = 306
7x +7y = 252
Doubling the second equation and subtracting the first, we have ...
2(7x +7y) -(3x +14y) = 2(252) -(306)
11x = 198
x = 18
y = 252/7 -x = 36 -18 = 18
A small box costs $18, as does a large box.