The area in the cross section is simply the difference in areas of the outer and inner circles. There are an infinite number of ways to get to this number if there is no other information regarding either circle or the relationship between the two.
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