Ballparking technique

As a general rule, ballpark first and calculate second. Naturally you should end up ballparking more at then end of the section and calculating more at the beginning, but it’s a good rule of thumb. Ballparking can take many forms. The first benefit to Ballparking is that you can’t do it if you don’t understand the question. The basic process of trying to come up with a ballpark range for an answer involves arriving at a conceptual understanding of what the question is asking. If you are at the tail end of a section, you might stop here and pick an answer.
If you are in your first ten, you might use this as a way of figuring out how to go about determining the actual answer.
Always, Ballparking is a valuable way to check your work. GRE questions tend to make sense. The correct answer to a question asking for the number of students in a class will not contain a fraction (ETS won’t generally chop a student in half). A question in which a person bicycles uphill one way and downhill on the way home, will not involve a distance greater than the distance a person could or would bike to work
in a day. If you are asked for time, and you know that the round trip of 20 miles took two hours, then each leg would average 60 minutes. If you are looking for the downhill leg, any answer greater than 60 is wrong and any answer less than the amount of time an average person could reasonably bike ten miles is wrong. This is Ballparking. It won’t necessarily eliminate four out of five wrong answers (although it could), but it will eliminate a few—and it will tell you the answer you generated actually makes sense.