the deck in hopes backyard is round.it has 5 posts evenly spaced around the edge to support a trellis.for her birthday party,she wants to connect each post to all the other posts with crepe-paper streamers.how many streamers will she need?
That would be C(5,2) = 5!/(2!3!) = 10
(This is the same kind of concept as "after a gathering of 5 people, they all shake hands with each other to say goodbye," how many handshakes are there )