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These are idioms...and idioms are annoying. They aren't really rules in the sense of having a logical reason. You're just supposed to memorize the usage.
These are correct:
Her efforts are aimed at increasing her GMAT score.
She is aiming for a specific result.
She is aiming at a specific target. (An actual, physical target—e.g., she's aiming her arrow at the target.)
She is aiming to increase her GMAT score. (person is aiming to VERB)
These are not:
Her efforts are aiming at increasing her GMAT score.
She is aiming to a specific result.
She is aiming at a specific result. ("aiming at" is usually referring to a physical thing)
She is aiming at increasing her GMAT score.
As I said, idioms are pretty much a "memorize this" situation. That's really annoying so I don't recommend spending a ton of time on idioms unless they are *very* common and very likely to be seen on the exam. You may or may not see this one, and SC questions typically give you multiple errors in the same problem...so I would concentrate on something else.
(Re: dedicated / dedicating—you can substitute "dedicated to" but not "dedicating to." Again...idioms.
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