The Quant section of the GMAT is not a math test. Really! It just looks like one on the surface. In reality, the test writers are testing us on how we think.
As such, they write many math problems in a way that hides what is really being tested or even implies a solution method that is not the best solution method. Assume nothing, and do not accept that what they give you is your best starting point!
Instead, slow down a little. First, just glance at the whole problem (before you really start reading) to see what kind of problem you have.
Next, read the problem and jot down any numbers, formulas, etc. Do not do any translation or simplification at this stage—in short, do not do any actual work yet. Just get the basics on paper, and wrap your brain around what the question is saying. You will be less likely to fall into their traps if you think before you act.
Then, reflect and organize: what have you got, and what should you consider doing with it? Do any pieces of information go together? Do you see any clues that give you an idea of how to solve the problem? Is the problem really obviously suggesting a certain path? Maybe that will work—but make a conscious decision that this really is your best path.
Most of the time, when an “obvious” path is suggested, some other path is actually faster or easier. Also, remember that your best approach might be to guess and move on, depending on how hard the question is.
Finally, if you are not going to guess, then get to it and work. You made some kind of plan during the previous step, so start working that plan!
If you get stuck at this stage, you are allowed to give yourself one chance to unstick yourself. Go back to an earlier step in your work to see whether you can find another way forward. If you find yourself still stuck, pick something and move on.
Want to see some examples of all this? Glad you asked. I have got a full two-part article for you with three different practice problems. Get to it, and let me know what you think!