Data Sufficiency 101:
In every class I teach, the reaction students have when they first encounter a data sufficiency problem is always the same. They are unsure of the correct approach, but feel they could do much better on such a problem if they had a firm grasp on the concept of data sufficiency.
Don’t work out the actual solution
To gain this grasp, we must start with the most basic idea of what exactly data sufficiency is asking you to do. A data sufficiency question is not centered around the actual solution. Rather, data sufficiency tests whether you are able to reach a single answer to the question.
You will be given a question and two statements. You must figure out if the information in the statements, alone or together, is sufficient to answer the question being asked. In other words, based on the statements could you solve the problem if you wanted?
Memorize the five answer choices
The five answer choices never change and all refer to which statement(s) are sufficient: Statement One but not Statement Two; Statement Two but not Statement One; the Statements together are sufficient but not alone; each Statement is sufficient on its own; or the Statements are not sufficient together nor alone.
Avoid doing more work than you need
This means that you can avoid doing quite a bit of work when encountering a data sufficiency problem. For example, once you have set up an equation, you have no reason to actually solve it as long as you can see that you could solve it enough to have a clear answer to whatever you are asked.
So, next time you see a data sufficiency problem, do not let it intimidate you. Rather, tell yourself that you are lucky to be seeing such a problem. Unlike the problem solving portion of the test, you may be able to avoid doing as much actual math. This, in turn, saves you time for problems that genuinely do take longer to complete and leads directly to a higher score.