Question: 1. How many edges does a k-regular graph with n vertices have? Explain.
2. At the Delicious Pie Bakery, there are 18 banana-containing pies, 23 chocolate-containing pies, 12 peanut-butter-containing pies, 15 chocolate-banana containing pies, 10 peanut-butter-chocolate-containing pies, and 3 chocolate peanut-butter-banana pies. There are 27 pies total. How many peanut butter-banana-containing pies are there?