One positive thing about not knowing the answer on the GMAT is that as a multiple choice test the correct response is listed. So, if you have an idea of what the answer should be and there is only one choice that fits, select it. It is best to use this method when you cannot figure out the problem and would otherwise get it wrong.
If you find have no clue about the right answer, you can work the problem backwards by using the given choices. To work the problem backwards, you will plug the answer choices into the problem, and see if you get the right answer. Take advantage of the fact that the GMAT organizes its answer choices in ascending order. First plug in the answer choice (C), which will be the middle value. If when you work the problem your answer is incorrect, you will see if the answer you got was too high or too low. This should tell you whether the correct answer is a value greater or less than C. With this information, you can select your next answer choice to plug into the problem. By working backwards a second time, you should be able to find the right answer.
When estimating and working backwards have not given you the correct answer, plugging your own numbers into an equation may help. This option is only viable for questions with unknown variables. Simply, choose your numbers to replace the variables in the problem's equation and skip the algebra. For example, if you're given a problem similar to "if x > 0, what is 20% of (6x-10) / 2?", simply let x = 2, and solve it. You will then see your answer in the answer choices in terms of the variable (x, in this case). Substitution allows you to solve a problem by using basic arithmetic instead of the more advanced algebra.