Under what circumstance can I get into a top 20 school with only a 640 GMAT?

Top 20 where? Globally or US or India or Madagascar?

If you are a from a under represented international community who is a ethnic minority and was suppressed in the past then it helps. If you have served in Army, or have accomplishments - like swimming in Olympics, or Start ups in silicon valley then be sure to highlight those as well.

Hey thanks for the hints. I'm looking at top 20 globally -pretty much everything through Ross in the FT rankings, although I could live with Duke or Esade. :)

Unfortunately I don't think my people have not been suppressed in the past and we are quite well represented in business school. I do have some accomplishments, I've been published and cited extensively in my field, graduated in the top 10 percent of my class, and have done some extensive community building volunteer work in Africa. Does that help?

@vby

Publishing and community work in Africa will certainly help. But that's (my guess) is material for 1 essay (leadership/positive influence), you gotta be outstanding in remaining of your essays (career goal, why mba. and the curve ball essay) so that adcom overlooks your GMAT. Might be difficult for top 20, but don't let that stop you from applying.

Got a 640 GMAT too but only have 1.5 years work experience. Can I get into George Washington?

As long as you have deep pockets, GW shouldn't be a problem. I visited them last year and they seem to attract a lot of Chinese who score high in GMAT (which boosts their rankings) and they got deep pockets to pay their high tuition fees.

Write good essays and I'm sure you'll be invited for interview!

Good luck!
P.S - Apply to their competitor schools as well, this will not only spread your risk and allow you some bargaining power come admission/scholarship time.

Thanks for that. I don't have deep pockets, unfortunately but I can afford the tuition.

Could you tell me who you consider to be the competitors?

If you cannot afford GW then where can you afford?

Sorry my English is not great. I meant that even though I do not have deep pockets, I could still afford to pay for GW. I was wondering about competitors so that I can apply to a couple different schools.

Consider thinking on these lines.

Geographically - GWU competes with American Univeristy - Kogod, Georgetown University, George Mason University, Howard University (attracts more African American crowd) in DC, and College of William and Mary, University of Virginia, University of Maryland and Johns Hopkins University in MD/VA/DC area.

Otherwise, GWU competes with similarly ranked schools - schools having same average GMAT and similar career outcomes like College of William & Mary etc.

Thanks for those suggestions. I think I might apply to William and Mary and American University as well, since they have lower GMAT scores for most candidates. While I would like to go to GW I don't know if my GMAT score is high enough. :)

Assuming you're applying for the 2017 intake, I think that with 1.5 years of work experience, the rest of your profile would have to be relatively exceptional to get into George Washington. Average work experience there is 5 years. Can you try for a higher GMAT score? Outscoring the average, at least, tends to help compensate for lower than average work experience. If you're aiming for 2018 then this would be less of a problem.