I believe its 45.
Its hard to explain but what I can say is if you subtract the percentage with the total of the skins you would subtract what you had from there and then you would have 45.
Step-by-step explanation:
-4x - 2y = 18
+ (4x - 4y = -36)
=> -6y = -18, y = 3
As we added the equations together, we have eliminated x and found y.
Hence the correct answer is "Add to eliminate x".