For a chain e-mail, each mailer sends messages to two new people. Some recipients continue the chain, and some don't. No one gets the same message twice. Sometime later, there are a total of 1500 message recipients. (That means your graph model will have 1501 vertices because the initiator of the chain e-mail is not one of the 1500 recipients.) How many people received messages but did not mail anything? Suggestion: Use the formulas Number of vertices = 2 × Number of parents + 1 and Number of vertices = Number of parents + Number of leaves.
