Another strategy that might get you to the answer faster than simply doing the problem outright, is plugging in numbers. This might make it easier to visualize the problem and help you work through it faster. If you are struggling with a GMAT problem and are having trouble visualizing the problem, try replacing the unknowns with a value. For example, take a look at this problem.
To practice law in their state, the third year law students at Western University have to pass the bar examination. If 1/3 of the class opted not to take the bar examination and 1/4 of those who did take the test, did so and failed. What percent of the 3Ls will be able to practice law in their state?
You could try to solve this problem outright, however an easier way might be to just pick a convenient number for the class and then use it to solve the equation. It’s important to choose your number carefully and strategically. Picking 5 for this problem wouldn’t work, because you can’t even take 1/3 out of 5 evenly. A better number is one that both 3 and 4 can go into, like 12. If the class has 12 people in it and 1/3 don’t take the test, that means 4 don’t take it and 8 do. Of those 8, 2 fail, so 6 in total are able to practice. The answer is asking for the percentage, so 6/12 is 50%.