Summation of elements in sublists

In my project I have various walls. Some contain the same materials. For further analysis I wish to calculate the totals for each material. For that reason I grouped the list by key. However, I am not certain how to proceed in calculating the material total in each of the sublists. Does anybody know a solution how to go about that?

Hello Tobias,

I would suggest this workflow!


Splendid. Thank you Vincent.