dividing by complete amount - how?

dividing by complete amount - how?

I want to sum up all contact with a specific source and then sum all earnings we made with this contacts and divide it by the sum of contacts. The required tables are called "Kunden/ Leads" and "Investments".
The Problem is if i say sum("Kunden/ Leads"."Created time")/sum("Investments"."Betrag im Vertrag") zoho reports counts only those contacts who have already done at least 1 investment. But i want to sum up ALL contacts not only those with an investment. 
I'm facing this problem many times. Always when i try to take all values from one field out of one table and do some maths with all the values of another field in another table zoho reports only sums up the matching part and ignores all other values.
Any tips?