Assuming the name of the column group is WeekStartDate, something like the following should work:

=iif(iif(isnothing(Sum(Fields!RMAs.Value,"WeekStartDate")),0,Sum(Fields!RMAs.Value)/Sum(Fields!RMAs.Value,"WeekStartDate"))=0,nothing,Sum(Fields!RMAs.Value)/Sum(Fields!RMAs.Value,"WeekStartDate"))

Note: the expression above is also eliminating divide by zero errors

Hope this helps!

-Marianne