Memorize a list of facts and formulas typically seen on the GRE to help you save time during the test. GRE study guides and online resources will have these lists available.
Avoid doing the algebra when you can. Try plugging the answer choices into simple equations. Start with answer choice C. The GRE discrete quantitative questions list possible answers in increasing or decreasing order. Starting with the middle value in answer C will let you eliminate incorrect higher or lower values.
Try adding two or more equations to get the result instead of solving each one individually. Some discrete quantitative questions do not require that you solve the equations given.
Replace the variables with a number on answer choices that use variables. The numbers 5 or 10 are easy to work with. Use the number to solve the given equation and to evaluate the answer choices.
Eliminate answer choices when you can. If you know that certain answer choices are too high or low, toss them out. Then you can focus on the feasible choices.
Practice answering GRE discrete quantitative questions. Use sample questions and practice to become familiar with the question type.