my answer is 8,901
The exact answer is 5697/640 or approximately 8.9. This is NOT a valid GMAT question because we are not expected to know the closed form evaluation of a geometric sum, nor would we be expected to add them up one by one.
IMO, I think we would be better served by problems that test skills we would need in the actual GMAT. Problem like this put pressure on readers to unnecessarily learn a formula that will not help them and possibly squeeze out one that they DO need to know.
Former Senior Instructor, Manhattan GMAT and VeritasPrep
Vice President, Midtown NYC Investment Bank, Structured Finance IT
MFE, Haas School of Business, UC Berkeley, Class of 2005
MBA, Anderson School of Management, UCLA, Class of 1993