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I received a PM asking me to clarify the difference between of and for.
There are no clearcut rules; prepositional modifiers serve many uses. That being said:

OF often serves to describe the NATURE of the noun being modified:
In a penalty OF one hundred dollars, the prepositional modifier describes the NATURE of the penalty.

FOR often serves to describe the CAUSE of the noun being modified:
In a penalty FOR speeding, the prepositional modifier describes the CAUSE of the penalty.

TO often serves to describe the PURPOSE of the noun being modified:
In a penalty TO deter crime, the prepositional modifier describes the PURPOSE of the penalty.

When prepositions change in the answers, the SC likely is testing an idiom.
The good news is that the current incarnation of the GMAT is placing much less emphasis on idioms.
Any answer choice that contains an idiom error is likely to contain at least one other error.
If you're not certain which preposition or idiom is needed in any particular instance, LOOK FOR A DIFFERENT ERROR.

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Mitch Hunt
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