$7.$ You buy a call option with a strike price of $$60$ and sell a call option with a strike price of $$55.$

a$)$ What does the gross payoff diagram look like for the portfolio of calls?

b$)$ If the price of the call with a $$60$ strike price is $$10,$ what do you know about the range of

acceptable prices for the call with a $$55$ strike price?