$ sum in field to be determined by no. of checkboxes picked

$ sum in field to be determined by no. of checkboxes picked

Hi

http://creator.zoho.com/rodrago/jo/form/1/

In the box "service je aufgabe" You can pick how many thing the customer wants to have done.

If he picks 1 in the field "Gesamtpreis" the value should be 75, two 150 and three 225.

How do I do that?