Since the posters in this thread are probably long gone (the posts are over 5 years old), the following advice isn't really for them.
This type of prompt involves some standard formula use and calculations - you'll do similar work on Test Day again and again. However, since the 5 answer choices were not included, we're essentially STUCK using that approach. Based on the wording of the prompt, the answers would have been numbers, which means that we could have TESTed THE ANSWERS and quite possibly answered the question in LESS time and with LESS work. During your practice, to maximize your overall abilities, you have to be comfortable using more than one approach. Take advantage of ALL of the information that you've been given (including what's in the answer choices) and you'd be amazed how "gettable" most GMAT questions actually are.
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