SSRS How to get the SUM of a row group that is itself an AVG

I have inherited a legacy SSRS Report that I am tweaking in Report Builder 3.0. A bunch of Detail rows are grouped in a Group row that displays the AVG of the Detail rows. Outside this is another Group Row that needs to display the SUM of the values in the inner Group row that is itself the AVG of the Detail rows.

Is this even possible? What would the expression look like?