Let's play a little GMAT guessing game. Take a look at the two GMATPrep screenshots below, and see if you can guess the quant score for each of these tests. (Sorry, the screenshots aren't beautiful, but I did my best to make them legible.) Keep in mind that the GMATPrep software uses exactly the same algorithm as the real GMAT test. On GMATPrep test #1, the student missed a total of 7 quant questions out of 37 (81% correct):
And on GMATPrep test #2, the test-taker missed a total of 21 questions out of 37 (43% correct):
Go ahead, take a guess. What quant score do you think these two students received on their GMATPrep tests? The lucky student who got 81% correct probably got a solid but imperfect score, right? And the poor schmuck who got 43% correct must have been vaguely suicidal after that GMAT quant disaster, yes?
Well, the first student got a perfect 51 on her quant section, and the second student earned a 44. (For what it's worth, he paired the 44 quant with a 41 verbal, for a grand total of 710, and he earned a very similar score on the real GMAT.)
Surprised? It turns out that a "perfect" GMAT quant score doesn't necessarily require complete perfection, and you can miss a ton of questions on the GMAT quant section without endangering your chance at a 700.
I discussed the GMAT scoring system in both a recent GMAT blog post (which profiled a mediocre math student who still earned a 720) and in an ancient, crusty GMAT blog post (which explored the fundamentals of the GMAT scoring algorithm), but I'll say it here again: your GMAT quant score doesn't really depend on how many questions you miss. It depends on which questions you miss.
As you probably already know, the GMAT exam "adapts" to your performance, selecting each question based on your answers to previous questions. Your final score is based primarily on the difficulty level of the questions you see on the test. If you see tons of hard questions, your score will be higher than if you see nothing but GMAT cream puffs.
By the time you reach, say, question #29, the GMAT scoring system already has 28 data points about your skills. If you get 27 of the first 28 questions correct--as student #1 did--you'll convince the GMAT scoring algorithm that you're pretty awesome. And it will spit out the hardest question it can find. Even if you miss that question, you'll still have missed only two out of the first 29, and the computer will spit out another really tough question.
So as you look at the student who earned a quant score of 51 while missing seven questions, you shouldn't be too shocked: six of her seven mistakes came at the end of the test, once the GMAT scoring system had "already made up its mind about her." All of the seven questions that she missed were unbelievably difficult, and the GMAT algorithm doesn't really punish test-takers for missing hard questions. Just ask Ms. A from this GMAT blog post.
Student #2 is admittedly an even more extreme case. He missed 21 questions and still scored above 700; that's not normal, but it's obviously possible under the right circumstances. In his case, he had a reasonably strong start to the test, missed only the toughest questions that the GMATPrep software threw at him, and managed to get just enough questions right to prevent a score meltdown. Again, this is a great illustration of the GMAT scoring system: you can miss piles of questions and still do well on the GMAT. You just can't afford to miss the easier questions, since those errors will send your GMAT score into a tailspin.
The bottom line is that a 51 isn't necessarily "perfect" on the GMAT, and a 700 doesn't necessarily require a high rate of accuracy. You can miss tons of hard questions and still do incredibly well on the GMAT, as long as you don't miss the questions that are relatively easy. And when you see impossibly difficult questions on your GMAT exam, just smile, and accept the fact that you can miss them without torpedoing your GMAT score.