Hi ziyuenlau,
Since the numbers involved in this prompt are relatively small, you can use 'brute force' to get to the correct answer without too much trouble.
We're told that there are two types of pencils: those that cost 24 cents and those that cost 30 cents. We're asked for the number of 24-cent pencils that Mary bought.
1) The total price of the pencils Mary bought was 102 cents
Notice that 102 cents is a relatively small total and there must be at least a few 24-cent pencils in that group (since the unit's digit of 102 is "2" and there's no way to get that "2" when taking a multiple of 30).
Increments of 24: 24, 48, 72, 96, 120
The only option that fits this information and that has a unit's digit of "2" is 72.... so there MUST be three 24-cent pencils and one 30-cent pencil.
Fact 1 is SUFFICIENT
2) Mary bought a total of 4 pencils
With this Fact, the answer can be 0, 1, 2, 3 or 4.
Fact 2 is INSUFFICIENT
Final Answer: A
GMAT assassins aren't born, they're made,
Rich
