Regarding Q 50, I should remind that I have an engineering in Turkey. As a Turkish engineer, the quantitative questions of GMAT is in secondary level. Therefore, I did not study GMAT quant. My only struggle was with Verbal part.
The major mistake that I did for the first two failures was not improving mystamina and studying different materials from OG. At first, I covered all recommended materials including; Kaplan 800, Princeton Review, Baron...
But no one was as effective as studying OG 11th and OG Verbal thourougly. I solved every question of these books with 75 minute intervals and I red each explanations of answers.
The other important point, I tested my progress with GMAT Prep. I solved these two tests many times, and every time, I experienced different questions. GMAT prep is a great indicater of your progress.