The deck in Hope's 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 report?

Answer 1

Each post is connected to 4 others. Since each streamer connects in both directions, you don't want to count them twice, so you have

5*4/2 = 10 streamers