Mary buys several pencils that cost 30 cents and 24 cents. How many of

==> In the original condition, it is a 2 by 2 question that appears most frequently on current gmat math.


According to the table above, there are 2 variables (x,y) and in order to match the number of variables to the number of equations, there must be 2 equations. Since there is 1 for con 1) and 1 for con 2), C is most likely to be the answer. By solving con 1) and con 2), you get 30x+24y=102, 5x+4y=17 and x+y=4, so you get x=1 and y=3. The answer is C. However, this question is an integer question, one of the key questions, so you use CMT 4 (A: if you get C too easily, consider A and B). For con 1), from 5x+4y=17, 5 and 4 are mutually exclusive and x and y are positive integers, you only get x=1, y=3, hence it is unique and sufficient.

Therefore, the answer is A, not C.
Answer: A