Comparing the 90% figure to the 50% figure is not an appropriate comparison; it's comparing apples to oranges.
The 90% figure is from randomly selected questions - in other words, some are very easy, some are easy, some are medium, some are hard, some are very hard.
The 50% figure is from an ADAPTIVE TEST - in other words, once the test learns you are pretty good at this, it intentionally tries to select questions that you have roughly a 1 in 2 chance of getting right, meaning you are probably seeing only hard questions. So of course you will get fewer right.
This doesn't mean the MGMAT questions you see are harder than what you see on the real test - in fact, it means that many of the OG problems you are doing are easier than what you will see on the real test because the GMAT will select certain questions just for you based on your performance.
Check around the forum for more info on the GMAT and adaptive testing! You might also want to check out the MGMAT Roadmap
for info on the difference between studying from the OG and taking a practice test.
Ryan Jacobs | Manhattan GMAT Instructor | San Francisco
Manhattan GMAT Discount | Manhattan GMAT Course Reviews | View Instructor Profile