Call options on a stock are available with strike prices of $$\$ 15, \$ 17 \frac{1}{2}$$, and $$\$ 20$$, and expiration dates in 3 months. Their prices are $$\$ 4, \$ 2$$, and $$\$ \frac{1}{2}$$, respectively. Explain how the options can be used to create a butterfly spread. Construct a table showing how profit varies with stock price for the butterfly spread.