C# LINQ Query

I have the following query:

var results = from theData in GeometricAverage group theData by new { study = theData.study, groupNumber = theData.groupNumber, GeoAverage= theData.GeoAverage } into grp select new { study = grp.Key.study, groupNumber = grp.Key.groupNumber, TGI = testFunction(grp.Key.GeoAverage, Also here I want to pass in the GeoAverage for only group 1 (but for each individual study)) }; 

What I want to do is that for each study, there are multiple groups with a GeoAverage figure for each group. The TGI is calculated by passing the GeoAverage figure for each group and the GeoAverage figure for group 1 (on each study) into the testFunction. I can't figure out how to pass in the value just for group 1.

Hope this makes sense.

EDIT: Sample of data:

Study Group GeoAverage 1 1 3 1 2 5 1 3 6 2 1 2 2 2 3 2 3 9 

So, for the above data, I would want each GeoAverage figure for each group, to be evaluated against the GeoAverage figure of group 1 within that same study. So if I have say a function:

int foo(int a, int b) { return a * b; } 

Using the data above, I would first evaluate study 1, group 1 against itself, so pass in GeoAverage 3 twice and return 9. For Study 1, group 2, pass in group 2 GA at 5, and that studys group1 GA at 3, returning 15.