What a cool little question.
To make this super easy, just use easy numbers.
Let's say the costs of the two articles are 30 and 20. The total cost is 50.
On the 30 one there is a loss of 40%.
-.4 x 30 = -12
On the 20 one there is a gain of 30%.
.3 x 20 = 6
The net loss is -12 + 6 = -6
Loss/Cost = -6/50 = -12%
The correct answer is A.
Alternative Approach
If the costs of the articles are in the ratio 3:2, then the cost of the first article is 3/5 of the total cost, and the cost of the second article is 2/5 of the total cost.
Loss of 40% of 3/5 = -1.2/5
Gain of 30% of 2/5 = .6/5
Net loss is -1.2/5 + .6/5 = -.6/5 = 12/100 = 12%
The correct answer is A.
