As far as I can tell after a quick check in FileMaker help,
GetSummary ( SummaryField ; SummaryField ) returns the same value as just referring to SummaryField in your calculation. It returns a "grand total" value.
You can compute an average using:
Let ( [ Q = SumAverageQuiz ;
T = SumAverageTest
Sum ( Q ; T ) / Count ( Q ; T )
If there is some reason to use GetSummary instead of just the summary field for which I am unaware, replace the summary field names used here with GetSummary function calls.
There are also a number of other ways to calculate these values using relationships or the ExecuteSQL function.
Yes thats just what I needed Thanks