With data sufficiency problems, be careful not to read any more into a statement than what is given. The main purpose of some difficult problems is to lure you into making an unwarranted assumption.If you avoid the temptation, these problems can become routine.
Example: Did Incumbent I get over 50% of the vote?
(1) Challenger C got 49% of the vote.
(2) Incumbent I got 25,000 of the 100,000 votes cast.
If you did not make any unwarranted assumptions, you probably did not find this to be a hard problem. What makes a problem difficult is not necessarily its underlying complexity; rather a problem is classified as difficult if many people miss it. A problem may be simple yet contain a psychological trap that causes people to answer it incorrectly. The above problem is difficult because many people subconsciously assume that there are only two candidates. They then figure that since the challenger received 49% of the vote the incumbent received 51% of the vote. This would be a valid deduction if C were the only challenger. But we cannot assume that. There may be two or more challengers. Hence, (1) is insufficient.
Now, consider (2) alone. Since Incumbent I received 25,000 of the 100,000 votes cast, I necessarily received 25% of the vote. Hence, the answer to the question is No, the incumbent did not receive over 50% of the vote. Therefore, (2) is sufficient to answer the question. The answer is B.