I like to think of promotions testing like going for a long distance run.  Your mind, like your body, can fatigue.  A critical factor that plays into testing is your concentration, which wears thin over time as you become mentally tired after focusing on a problem for too long.  Like a long distance run, you have to pace yourself.  Go through the test, answering the easier questions first and skip the harder questions.  Use that scrap piece of paper to note the skipped questions and be sure you mark the appropriate question on your score sheet for the ones you answer.  It would be a shame if you meant to mark an answer for question 10 and accidentally marked the circle on your score sheet for question 9.  This method has helped me focus my mental energy where it counts and has shifted the bulk of the testing time on answering the toughest questions.