A simple modus operandi over-invoicing of imports and under-invoicing of exports could have led to the flight of a mind- boggling $15.7 billion (Rs 57,000 crore) from India to the United States between 1993 and 1995. So concludes a study by three US- based economists: John S. Zdanowicz, William W. Welch and Simon J. Pak of the Florida International University, Miami.