NOTE: Shortcuts in Algebra
Keep in mind that the GMAT is NOT a standard achievement test. The test measures your resourcefulness and intellect in solving problems, not the methods achieved to do so. This means that you should use shortcuts whenever you can.
This means that you should freely use ALL THE ADVANCED TECHNIQUES, such as Plug in, Backsolving and Experiments (see math introductory chapter). Some GMAT questions are designed to be solved this way.... and doing the conventional algebra will take too much time. Don't be afraid to start Backsolving or Plugging in as soon as you hit a brick wall.