Counting rows from linked Table

Hi all,

This is my setup...
I have 1database, with 4 tables. The 4 tables are needed, because they contain all different kind of data, but they have to be merged in one report.
One table is the information about an activity
One table about the participants
One table with the expenses and one with the income.

So I have 2 Details Sections, with in each a subreport (expenses, income)...that runs well. The get their data out of their respective database.
Now I want to loop through another table (participants) to get the amount of participants there were. In this table I have an Act_ID, which refers to the ID in the activity-table.
When I run a Summary/Running Total Formula...he gives me the exact number, but all detail section have been copied...the same amount as the exact amount...I think he's looping...
How can I solve this?

Thanks