yogi_Compute Number Of Students Per Class And Number Of Sections For Each Class

Yogi Anand, D.Eng, P.E.      Google Spreadsheet   Post  #821  Oct 23, 2012    www.energyefficientbuild.com.

user Monty2012 said: (http://productforums.google.com/forum/?zx=jro1h3a5xnh2#!category-topic/docs/spreadsheets/1i7EMI-AyE0)
Sumif formal that only counts a unique criteria one time
I suspect this is an easy fix, but I think I've been over thinking it for too long.

I need a formula that will sum the number in Column A (Number of Students)  based on Criteria from Column B (Title of the Class), but I only want it to add the number once based on data from Column C (Class Section Number).  

My goal:  From a list of many class (ENGL, EDUC, POLI SCI) types, I want to know how many students from ENGL 101 attended a presentation, but some ENGL sections attended the presentation twice. I need to prevent double counting the students in the section that attended twice.

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Here is a sample spreadsheet w/my expected results:  https://docs.google.com/spreadsheet/ccc?key=0As1jqHN8FeKsdGtCYlpCTnB6d2QxemV5RUV3RnlPa2c

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following is a solution to the problem

please check your posted results ... with mine and let me know