In a retail shop there are 21000 different types of items offered for sale. In the shop can enter customers only in pairs of two. From each pair, one person randomly select 3 items to purchase and the other 6 items. If at least one item is chosen in common by both persons in the pair, then it is considered a successful outcome.
Question: After 300 pairs of customers made their purchases, what is the expected number of successful outcomes?