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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.

Guest May 28, 2017
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I would also like to see someone answer this question.

I have not seen a question like this before.

Melody  May 29, 2017
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Vertice is any node in the graph.

Parent is a vertice that has at least one path out of it leading to another vertice.

Leaf is a vertice that is not a parent.

let P be the number of parents and L the number of leafs. The folks that didn't forward the email are the leafs.

V=1501 = 2P+ 1

P=750

V=1501 = P + L

L = 751

I still need to think about why this always works ...

Melody  May 30, 2017