The easiest way to get averages from the grade book for a particular "category" of assignment is by Score Sheet Report export
Select the Score Sheet Report from the reports tab by double clicking. (use the settings below)
Then click run report button lower right, and select save the report, saving to your H: drive
Then Open the Excel Spread Sheet by going to the H: drive and Double Clicking on the file.
Then you can delete any unwanted columns by right clicking on the letter of the column and selecting delete.
To get the average, in an empty cell type =average(
Then Hi-lite the row of cells to be averages
the Type ) to finish the calculation.
When its complete the cell should hold the average
You can copy the average to the rest of the cells down the page by
hovering the mouse over the right lower black box until the cursor becomes a solid black cross
the click and drag down the column, this will copy the formula to all the lower cells.