I think that the correct answer is E, because we do not have any clue of the profit of each type of card.
Forget conventional ways of solving math questions. In DS, Variable approach is the easiest and quickest way to find the answer without actually solving the problem. Remember equal number of variables and independent equations ensures a solution.
A souvenir shop made $ 2,400 in revenue selling postcards. If a large postcard costs twice as much as a small postcard, the shop sold 950 large postcards, and it sold no other type of postcard besides these two sizes, then how many small postcards did it sell?
(1) A large postcard costs $ 2.
(2) If the shop had sold 20% fewer small postcards, its revenue would have been reduced by 4 1/6 %.
When it comes to a question like the above, the question is frequently given in the Gmat Math test, which is "2 by 2" like the table below.
Attachment:
GCDS Bunuel A souvenir shop made (20151215).jpg
In this question, there are 4 variables(a,b,c,d) and 3 equations(b=2a, b=950, and ca+db=2,400), which should match with the number of equations. So, you need 1 more equation. For 1) 1 equation, for 2) 1 equation, which is likely to make D the answer.
1) c=2 is unique, which is sufficient.
2) c(0.8a)+db=2,400(1-4 1/6 %) is sufficient, which is sufficient. Therefore, the answer is D.
-> For cases where we need 1 more equation, such as original conditions with “1 variable”, or “2 variables and 1 equation”, or “3 variables and 2 equations”, we have 1 equation each in both 1) and 2). Therefore, there is 59 % chance that D is the answer, while A or B has 38% chance and C or E has 3% chance. Since D is most likely to be the answer using 1) and 2) separately according to DS definition. Obviously there may be cases where the answer is A, B, C or E.