You're organizing a party for which you send out 60 invites, saying each invitee is allowed to bring one additional guest if they wish. Suppose it is equally probably at the invitee herself does not show up, shows up alone, or shows up with a guest.
What is the expected number of guests that might turn up at your party, and its standard deviation?
What ishe probability the number of guests is between 70 and 80?