$1.$ Call options on a stock are available with strike prices of $$15,$ $$17\mathrm{.}5$ and $20$$ before expiration date. The call premiums for each are $$4,2$$$,$ and $$0\mathrm{.}5,$ respectively. Explain how the options can be used to create a butterfly spread. Can you explain how do on exsel with spreadshit.