OK - let's set up the Rollup summary for the number of VIP tickets sold.
1) Tickets Purchased field type in the Attendees object must be Number.
2) Attendees object must be related to the Festivals object using the Master-Detail relationship type
3) Create a new field on the Festivals object.
4) Field type should be Rollup Summary
5) In Step 3 where you are defining the new customer field confirm your settings match the screen shot below.
6) Since we are summarizing VIP tickets sold Field To Aggregate must be Tickets Purchased.
