Wrong result for sum of value of Purchase Order Items

Hi everyone, I am analyisng a P2P process and in my data model I have an activity table, which is linked to the tables Vendor and Position via a vendor-/position-number. The Vendor table contains master data for all vendors which I buy stuff from, i.e. one row for each vendor. And the Position table contains master data regarding the purchas-positions I buy, i.e. one row for each purchase-position. So there is a 1:N relationship between the vendor- / position-table (1) and the activity table (N). Now I would like to display the amount I pay for all positions bought at one vendor. But if I sum up the values for all positions belonging to a certain vendor i do not get the desired value, but I get the sum for all positions for the vendor multiplied with all activities which have been carried out for this position. E.g. if I have 2 items I bought from a vendor, each costs 10, the formula does not calculate 20, but 180, as for both items 9 activites have been carried out. (10 * 9 * 2 = 180). Does anybody know a solution to this problem? Thanks a lot already in advance! Greets Jonas